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Additional Water for the San Joaquin River Agreement,
2000 – 2010 Supplemental EIS/EIR

4. SURFACE WATER RESOURCES

Index

4.1 AFFECTED ENVIRONMENT

This section of the Supplemental Environmental Impact Statement/Environmental Impact Report (SEIS/EIR) focuses on the surface waters of the San Joaquin River Basin and the resulting flows from the basin into the Delta as measured at the U.S. Geological Survey gauging station on the San Joaquin River near Vernalis. Details of the surface water resources in the study area are described in Meeting Flow Objectives for the San Joaquin River Agreement, 1999-2010, Final Environmental Impact Statement/Environmental Impact Report (Final EIS/EIR, Reclamation 1999). This section contains a summary of the water resources in the study area with emphasis on those resources directly related to the acquisition of "supplemental" water.

The San Joaquin River Basin is contained within the southern portion of the Central Valley of California. The basin extends approximately 250 miles north-to-south, encompasses about 32,000 square miles, and is bounded by the Sierra Nevada mountains on the east and the Diablo Range on the west. Extensive water supply, hydroelectric, and flood control efforts during the past century have resulted in the construction of dams and reservoirs that now control the flow on nearly all major streams in the San Joaquin River Basin.

4.1.1 Surface Water in the San Joaquin River Basin

The primary sources of surface water to the San Joaquin River Basin are rivers that drain the western slope of the Sierra Nevada Range. Each of these rivers, the San Joaquin, Merced, Tuolumne, Stanislaus, Calaveras, Mokelumne, and Cosumnes, drains large areas of high elevation watershed that supply snowmelt runoff during the late spring and early summer months. Historically, peak flows occurred in May and June and flooding occurred in most years along all of the major rivers. The only rivers considered in this document as sources of supplemental water are the Merced and the Tuolumne rivers. The other rivers in the basin are not discussed. The Final EIS/EIR provides additional information.

4.1.2 Upper San Joaquin River and Tributaries

The San Joaquin River originates in the Sierra Nevada at an elevation above 10,000 feet and flows into the San Joaquin Valley at Friant. Along the valley floor, the San Joaquin River receives additional flow from the Merced, Tuolumne, and Stanislaus rivers. Flows in the upper San Joaquin River are regulated by the Central Valley Project (CVP) Friant Dam, which was completed in 1941 to store and divert water to the Madera and Friant-Kern canals for irrigation and municipal and industrial water supplies in the eastern portion of San Joaquin Valley. Releases from Friant Dam are generally limited to those required to satisfy downstream water rights. Millerton Lake, formed by Friant Dam, has a capacity of 520,000 acre-feet.

4.1.3 Lower San Joaquin River and Tributaries

The lower San Joaquin River is the section of river from the confluence with the Merced River (below Fremont Ford) to Vernalis, which is generally considered the southern limit of the Delta. It is characterized by the combination of flows from tributary streams, major rivers, groundwater accretions, and agricultural drainwater. The drainage area of the San Joaquin River above Vernalis is approximately 13,356 square miles. However, little water is contributed from the upper San Joaquin River, except during flood events. Therefore, flows in the lower San Joaquin River are primarily governed by the tributary inflows from the Merced, Tuolumne, and Stanislaus rivers.

4.1.3.1 Merced River

The Merced River drains an area of approximately 1,273 square miles east of the San Joaquin River, and produces an average unimpaired runoff of approximately 1 million acre-feet. The major water supply reservoir on the river is Lake McClure, with a capacity of 1,024,000 acre-feet. It is formed by New Exchequer Dam, completed in 1967, which regulates releases to the lower Merced River. New Exchequer Dam is owned and operated by the Merced Irrigation District (Merced ID) for power production, irrigation, and flood control.

4.1.3.2 Tuolumne River

The Tuolumne River drains a watershed of approximately 1,540 square miles, and produces an average annual unimpaired runoff of approximately 1.8 million acre-feet. Flows in the lower portion of the Tuolumne River are controlled primarily by the operation of New Don Pedro Dam, which was constructed in 1971 jointly by the Turlock Irrigation District (TID) and the Modesto Irrigation District (MID) with participation by the City and County of San Francisco (CCSF). The 2.03-million-acre-foot reservoir stores water for irrigation, hydroelectric generation, fish and wildlife enhancement, recreation, and flood control purposes.

4.1.3.3 Stanislaus River

The Stanislaus River drains a watershed of approximately 900 square miles, and produces an average unimpaired runoff of approximately 1.056 million acre-feet. Flows in the lower Stanislaus River are controlled by releases from the New Melones Reservoir, which has a capacity of 2.4 million acre-feet, and is operated by the U.S. Bureau of Reclamation (Reclamation) as part of the CVP. The main water diversion point on the Stanislaus River is Goodwin Dam, which provides for delivery to Oakdale and South San Joaquin irrigation districts.

4.1.3.4 San Joaquin River at Vernalis

Flows in the San Joaquin River at Vernalis are affected by the operation of upstream facilities on the San Joaquin, Merced, Tuolumne, and Stanislaus rivers, as well as by deliveries to the Mendota Pool from the Delta-Mendota Canal and overflows from the Kings River in the Tulare Lake Region. Prior to the construction of major dams on the San Joaquin River and its tributaries, average monthly flows peaked during May and June in response to snowmelt runoff. Unrestricted flows have not occurred since the construction of the original Exchequer and Don Pedro reservoirs in the 1920s. Flows in the San Joaquin River near Vernalis have been further altered from natural conditions by the completion of Friant Dam in 1941, New Exchequer Dam in 1967, New Don Pedro Dam in 1971, and New Melones Dam in 1978. Since 1978, average monthly flows in the San Joaquin River at Vernalis have been more uniform throughout the year, with maximum flows less than historical levels.

4.1.4 Surface Water Quality in the San Joaquin River Basin

Surface water quality in the San Joaquin River Basin is affected by several factors, including natural runoff, agricultural return flows, biostimulation, construction, logging, grazing, operations of flow regulating facilities, urbanization, and recreation. In addition, irrigated crops grown in the western portion of the San Joaquin Valley have accelerated the leaching of minerals from soils, altering water quality conditions in the San Joaquin River system.

Water quality in the San Joaquin River varies considerably along the stream’s length. In the reaches above Millerton Lake, water quality is generally excellent. However, there are several reaches of the river below Friant Dam that are often dry due to the groundwater percolation. From Salt Slough to Fremont Ford, most of the flow in the river is derived from irrigation returns carried by Salt and Mud sloughs. This reach of the San Joaquin River typically has the poorest water quality of any reach of the river.

As the San Joaquin River progresses downstream from Fremont Ford, water quality generally improves at successive confluences, specifically at those with the Merced, Tuolumne, and Stanislaus rivers. In the relatively long reach between the Merced and Tuolumne rivers, however, mineral concentrations tend to increase due to agricultural drainwater return flows, other wastewaters, and effluent groundwater (DWR 1965 as cited in Reclamation 2000a). Total dissolved solids (TDS) in the San Joaquin River near Vernalis has historically ranged from 52 milligrams per liter (mg/L) (at high stages) to 1,220 mg/L during the period from 1951 to 1962 (DWR 1965 as cited in Reclamation 2000a). In 1978, the State Board included a provision in Water Right Decision 1422 (State Board 1978) that Reclamation maintain average monthly concentrations of TDS in the San Joaquin River at Vernalis of 500 mg/L as a condition of the operating permit for New Melones Reservoir on the Stanislaus River. In the Water Control Quality Plan for the San Francisco Bay and Sacramento-San Joaquin Delta Estuary (1995 WQCP, State Board 1995a) the water quality objective is expressed as a seasonal 30-day running average value of electrical conductivity. For the period April through August, the objective is 0.7 mmhos/cm and the objective is 1.0 mmhos/cm the remainder of the year. (This corresponds to TDS concentrations of 427 and 610 mg/L.)

4.1.5 Water Facilities and Operations

4.1.5.1 Central Valley Project Facilities and Operations

The CVP is the largest surface water storage and delivery system in California, with a geographic scope covering 35 of the state's 58 counties. The project includes 20 reservoirs, with a combined storage capacity of approximately 11 million acre-feet; 8 powerplants and 2 pump-generating plants, with a combined generation capacity of approximately 2 million kilowatts; 2 pumping plants; and approximately 500 miles of major canals and aqueducts. The CVP supplies water to more than 250 long-term water contractors in the Central Valley, the Santa Clara Valley, and the San Francisco Bay Area.

Historically, approximately 90 percent of the CVP water has been delivered to agricultural users, including senior water rights holders. Total annual contracts exceed 9 million acre-feet, including over 1 million acre-feet of Friant Division Class II supply, which is generally available only in wet years.

CVP operations are influenced by a myriad of general operating rules, regulatory requirements, and facility-specific concerns and requirements. A summary of the operations of the CVP is provided in the Final EIS/EIR (Section 3.2.5.1, Reclamation 1999) and is not repeated here.

The Eastside Division of the CVP includes water storage facilities on the Stanislaus River (New Melones Dam, Reservoir, and Powerplant), Chowchilla River (Buchanan Dam and Eastman Lake), and Fresno River (Hidden Dam and Hensley Lake). All of the dams and reservoirs in this division were constructed by the U.S. Army Corps of Engineers. Upon its completion in 1978, the operation of New Melones Dam was assigned to Reclamation to provide flood control, satisfy water rights obligations, provide instream flows, maintain water quality conditions in the Stanislaus River and in the San Joaquin River at Vernalis, and provide deliveries to CVP contractors.

The operating criteria for New Melones Reservoir are governed by water rights, instream fish and wildlife flow requirements, instream water quality, Delta water quality, CVP contracts, and flood control considerations. Flows in the lower Stanislaus River serve multiple purposes. These include providing water for instream water rights obligations, meeting instream fishery flow requirements, maintaining instream water conditions of dissolved oxygen, and maintaining water quality conditions in the San Joaquin River at Vernalis, in accordance with Water Right Decisions 1422 and 1641, and the 1995 WQCP. Water is also released from New Melones Reservoir to meet, to the extent available, the San Joaquin River flow requirements in the 1995 WQCP.

During the period from 1987 to 1992 New Melones Reservoir was drawn down to approximately 80,000 acre-feet. Many Stanislaus River stakeholder meetings were convened to coordinate management of limited water supplies. Consequently, the May 1997 New Melones Interim Operations Plan provides examples of operation under the 50 percent (most probable) and 90 percent probability of exceedance (90 percent chance of having increased flows) hydrologic conditions which include Water Years 1997 and 1998 (Reclamation 2000k). This interim plan is currently the subject of stakeholder refinement.

4.1.5.2 State Water Project Facilities and Operations

State Water Project (SWP) facilities consist of 22 dams and reservoirs that capture and store water on the Feather River, in order to deliver water to service areas in the Feather River Basin, the San Francisco Bay area, the San Joaquin Valley, the Tulare Basin, and Southern California. Lake Oroville, SWP’s largest reservoir, with a storage capacity of approximately 3.5 million acre-feet, regulates the Feather River for release to Sacramento River and the Delta. The water is diverted by various facilities of the SWP for delivery to contractors or salinity control. In addition, Lake Oroville provides flood control, fish and wildlife habitat, irrigation water, hydroelectric generation, and recreation (Water Education Foundation 1997).

The SWP operates two diversion facilities in the Delta. The North Bay Aqueduct diverts water from the north Delta near Cache Slough for agricultural and municipal uses in Napa and Solano counties. In the southern portion of the Delta, the Banks Delta Pumping Plant lifts water into the California Aqueduct from the Clifton Court Forebay. The California Aqueduct is the state's largest and longest water conveyance system (444 miles), beginning at

the Banks Pumping Plant and extending to Lake Perris south of Riverside, in Southern California. Water in the California Aqueduct flows to O’Neill Forebay, from which a portion of the flow may be lifted to the joint CVP/SWP San Luis Reservoir for storage. From O’Neill Forebay, the joint-use portion of the aqueduct, San Luis Canal, extends south to the southern end of the San Joaquin Valley. The SWP portion of the aqueduct continues over the Tehachapi Mountains to the South Coast Region.

About 30 percent of SWP water is used for irrigation, mostly in the San Joaquin Valley. Approximately 70 percent is used for residential, municipal, and industrial use, mainly in southern California. Currently, the SWP has contracted a total of 4.22 million acre-feet for delivery in San Joaquin River Region, the Central Coast Region, and the San Francisco and South Coast regions. Of this amount, about 2.5 million acre-feet is designated for the Southern California Transfer Area, nearly 1.36 million acre-feet to the San Joaquin Valley, and the remaining 0.37 million acre-feet to the San Francisco Bay area, the Central Coast Region, and the Feather River area.

4.1.5.3 San Joaquin River Group Authority Willing Sellers

Supplemental water for the Project would come from willing sellers on either or both of the Merced or Tuolumne rivers, up to a maximum of 47,000 acre-feet from one or both rivers. Districts willing to supply supplemental water for the Vernalis Adaptive Management Plan (VAMP) Target Flow are the Merced ID, MID, and TID. A brief discussion of these districts is provided below.

Tuolumne River - Modesto and Turlock Irrigation Districts

Tuolumne River water has been used for irrigation in the Central Valley since the late 1890s, when MID/TID constructed storage and conveyance facilities. The water resources of the upper basin were developed for water supply by the CCSF during the early decades of the 20^th century. This development modified the natural flow regime of the Tuolumne River; on average, more than 60 percent of the annual flow has been diverted for agricultural or municipal and industrial use. The three reservoirs in the Hetch Hetchy System (not including storage in CCSF’s Don Pedro Water Bank) can store up to 35 percent of the mean annual unimpaired flow of the river as measured below La Grange. New Don Pedro Reservoir, along with the two smaller district-specific storage reservoirs (Modesto Reservoir and Turlock Lake), can store up to 112 percent of the mean annual flow of the river (FERC 1996).

The New Don Pedro Project (NDPP) is owned and operated by the Districts and consists of the New Don Pedro Dam, Don Pedro Reservoir (New Don Pedro Lake), and the New Don Pedro Powerhouse. Diversions from the NDPP system as well as from La Grange Dam, the TID and MID diversion facilities at the La Grange Dam, the TID canal system, TID’s Turlock Lake, the MID canal system, and MID’s Modesto Reservoir currently supply water to agricultural users. MID began supplying Tuolumne River water to the city of Modesto in 1995. NDPP also provides flood control, hydropower production, reservoir-based recreation, and fish and wildlife conservation.

The New Don Pedro Dam is an earth and rockfill structure located at Tuolumne River mile 54.5. It has a crest length of 2,300 feet and a maximum height of 585 feet above the streambed. The dam was constructed in 1971 to replace the original Don Pedro Dam, which was located approximately 2 miles upstream. The New Don Pedro Reservoir has a gross capacity of 2,030,000 acre-feet and a net usable capacity for irrigation, flood control, and hydropower generation (FERC 1996).

La Grange Dam is a diversion facility also owned and operated by the Districts and is located 2.3 miles downstream of New Don Pedro Dam. The dam is a 130-foot-high overflow structure built in 1893 and impounds approximately 500 acre-feet of water. The Turlock Main Canal and the Modesto Main Canal divert water from just above La Grange Dam into an extensive network of irrigation canals on both sides of the Tuolumne River, with TID’s canals on the south and MID’s on the north. Both irrigation districts have an intermediate storage reservoir at the upper end of their canal network to help regulate flows. Turlock Lake, on the south side of the Tuolumne River, has a capacity of 48,000 acre-feet. Modesto Reservoir, on the north side of the river, has a capacity of 28,000 acre-feet (FERC 1996).

The Hetch Hetchy System is owned and operated by CCSF and is not part of the FERC-licensed NDPP facilities. CCSF regulates the upper portion of the Tuolumne basin through the operation of its Hetch Hetchy System for municipal and industrial water supply and hydropower generation. Hetch Hetchy facilities include O’Shaughnessy Dam at the Hetch Hetchy Reservoir; the Hetch Hetchy Aqueduct; the Canyon Power Tunnel; the Kirkwood Powerhouse; the Early Intake Diversion Dam; and the Cherry Creek Power Development. A portion of the storage in New Don Pedro is allocated to CCSF through a water bank arrangement (FERC 1996).

Merced River - Merced Irrigation District

Flows in the Merced River are regulated primarily by four major facilities: New Exchequer Dam (Lake McClure), McSwain Reservoir and Powerplant, Merced Falls Dam, and Crocker-Huffman Dam. The largest reservoir, Lake McClure, has a capacity of 1,024,000 acre-feet and is created by New Exchequer Dam. Merced ID owns and operates the reservoir for power, irrigation, recreation and flood control purposes. Water released from Lake McClure passes through a series of powerplants and smaller diversions before reaching the main diversion point. McSwain Reservoir, a part of Merced ID’s Merced River Development Project, serves as an afterbay to New Exchequer Dam and Powerplant, re-regulating power generation releases to the Merced River. The capacity of McSwain Reservoir is 9,200 acre-feet (Merced ID 1997). Merced Falls Dam, from which diversions are made to Merced ID’s Northside Canal, is owned and operated by PG&E. Downstream of Merced Falls Dam is the Crocker-Huffman Dam, from which diversions are made to Merced ID’s Main Canal. On average, just over 50 percent of the annual streamflow in the Merced River below Merced Falls Dam is diverted to Merced ID’s Main Canal (500,900 acre-feet per year between 1955 and 1980) (Merced ID 1997).

4.2 ENVIRONMENTAL CONSEQUENCES AND MITIGATION MEASURES

This section evaluates the impacts of providing supplemental water for VAMP during "double-step years" as described in Chapter 2 of this SEIS/EIR on surface water resources. During "double-step" years, up to 47,000 acre-feet of supplemental water (in addition to the 110,000 acre-feet currently available) could be needed to provide full VAMP test flow conditions at Vernalis. It was assumed in this analysis that all of the supplemental water

would be obtained from either the Tuolumne or the Merced river. Water could be obtained from both rivers in any particular year but would not exceed 47,000 acre-feet in total.

4.2.1 Key Impact Issues and Evaluation Criteria

To evaluate the effects of supplying supplemental water for VAMP on water deliveries, carry-over storage, and flows/water quality, two key parameters are analyzed: changes in carry-over storage and changes in river flows. These represent the major water supply parameters affected by implementation of the Project alternatives. Because of the way in which "double-step" years are calculated, they would only occur when either the existing year or the previous year is relatively above normal in wetness. Analysis records that carry-over storage would be available.

Key issues considered in this analysis are:

• Effects on water deliveries to current water users in the affected districts

• Changes in carry-over storage in New Don Pedro Reservoir and Lake McClure

• Flow changes in the Merced and Tuolumne rivers

• Impact on the New Melones Project’s ability to meet water quality objectives at Vernalis, including the impacts associated with a different release pattern/timing of releases caused by the acquisition of additional water on the tributaries

For carry-over storage, changes of less than 10 percent (between the Proposed Action and No Action) are considered less than significant.

4.2.2 Analysis/Modeling Methodology

The full hydrologic analysis is provided in Appendix A. This analysis was conducted to evaluate a range of potential hydrologic effects attributable to the Proposed Project/Action on the Tuolumne and Merced rivers. Two operational settings were developed: No Action and the Proposed Project/Action.

• The No Action Alternative represents existing hydrology and operations within the Bay-Delta watershed, including the CVP and SWP meeting the 1995 WQCP and Biological Opinions on Winter-run Chinook salmon and Delta Smelt. This alternative also includes the current implementation of Water Right Decision 1641 (State Board 1999) inclusive of the San Joaquin River Agreement (SJRA) as capped by the provision of up to 110,000 acre-feet of VAMP water. Operations for the San Joaquin River include Reclamation operating New Melones Reservoir to the Interim Plan of Operation (Reclamation 1997b).
  
  
• The Proposed Project/Action Alternative represents the performance of the SJRA if, during VAMP double-step years, the full amount of water is provided to achieve the VAMP Target Flow. The hydrology of the Proposed Project/Action (up to 47,000 acre-feet of supplemental water) is evaluated against the hydrologic setting of the current implementation of the SJRA (the No Action setting).

The SJRA has a term of 12 years (unless extended); however, the hydrologic character of the 12 years cannot be predicted. To evaluate the range of conditions and hydrologic impacts that may occur, the Proposed Action is evaluated using the long-term hydrologic sequence of the period 1922 through 1992. Within that period of historical record, various sequences of hydrologic events occurred ranging from flood to extended periods of drought. During this modeled period, there are many years when the Proposed Action is not needed, since the VAMP Target Flow can be achieved with the current implementation of the SJRA. During approximately 10 percent of the historical record, the Proposed Action is needed. Details of the modeling methodology are provided in Appendix A.

4.2.2.1 Operation Simulation Models

The hydrologic analysis (Appendix A) was based on the most up-to-date models available in November 2000. This analysis relied on the interface of three hydrologic models to simulate the potential hydrologic effects of the Proposed Project/Action, SANJASM, STNMODAM (Stanislaus Operations Model), and PROSIM.

San Joaquin Area Simulation Model (SANJASM)

Reclamation’s SANJASM model provided the simulation of the San Joaquin River upstream of the confluence of the Stanislaus River, including the hydrology of west-side San Joaquin Valley CVP deliveries.

Stanislaus Operations Model (Modified Version)

A modified version of Reclamation’s STNMODAM spreadsheet model provided the simulation of Stanislaus River operations under assumptions of Reclamation’s Interim Plan of Operation for New Melones.

Projects Simulation Model (PROSIM)

Reclamation’s PROSIM provided the simulation of the CVP and SWP, and the Bay-Delta.

Within the context of the two alternatives, the No Action and Proposed Project/Action, eight simulations were performed. The model uses average monthly hydrologic data and a monthly time-step. Although the VAMP 31-day test flow period could be established anytime during the April through May period, the No Action and the Proposed Action were each assumed to occur during the entire month of April or May. Two simulations were therefore needed to determine the required VAMP Target Flows, one for the April pulse flow assumption and a second for the May pulse flow assumption. Similarly, two simulations were needed to model the No Action Alternative incorporating the provision of up to 110,000 acre-feet of VAMP flow. Another four simulations represent the scenario where the entire supplemental flow (up to 47,000 acre-feet in excess of the 110,000 acre-feet) originates entirely from the Tuolumne River or entirely from the Merced River, either occurring in April or May.

4.2.2.2 Modeling Assumptions – No Action Alternative

The No Action Alternative is the same as existing conditions. Before simulating the No Action Alternative, the VAMP Target Flows were determined. After determining the VAMP Target Flow levels, the flow division among the members was estimated based on a series of procedures discussed in the hydrology analysis (Appendix A). This provided the basis for comparison with the Proposed Action.

4.2.2.3 Modeling Assumptions – Proposed Action Setting

After determining the water to be provided by the San Joaquin River Group Authority members under the basic provisions of the SJRA (No Action, which represents existing conditions), supplemental VAMP releases required in excess of 110,000 acre-feet were modeled.

For the model runs assuming that up to 47,000 acre-feet of supplemental water would originate from the Tuolumne River, it was assumed that the MID and TID provided additional stream releases, in this case from New Don Pedro Dam and passed at La Grange Dam. This release would be an increase in flow above the release that would otherwise be made in the absence of the Proposed Project/Action (i.e., the release occurring in the No Action setting).

For the model runs assuming that up to 47,000 acre-feet of supplemental water would originate from the Merced River, it was assumed that increased stream releases would come from Lake McClure/New Exchequer Dam. This release would be an increase in flow above the release that would otherwise be made in the absence of the Proposed Project/Action.

4.2.3 Environmental Impacts and Mitigation

This section evaluates the effects of the releases on the Tuolumne and Merced rivers on water deliveries, carry-over storage, and changes in flow and water quality on the San Joaquin River. The potential impact of the Proposed Project/Action manifests as reduced storage in New Don Pedro and/or New Exchequer Reservoirs until such time that the storage is recovered through reductions and releases from the reservoirs that would otherwise be in excess of instream flow requirements. The Proposed Project/Action is not anticipated to affect the diversion or delivery of water to any water user within the affected basins; therefore, no change to the timing or occurrence of return flows is anticipated.  Table 4-1 presents a summary of the changes in flow and reservoir storage for both the April and May conditions on the Tuolumne and Merced rivers, based on the data contained in the hydrology analysis (Appendix A).

4.2.3.1 Water Deliveries

MID, TID, and the Merced ID make water deliveries to agricultural and municipal/industrial users.

No Action

Water deliveries under the No Action Alternative would be the same as existing conditions. Water that could be used for the supplemental up to 47,000 acre-feet would remain in storage until released at a future date for hydroelectric purposes, agriculture, municipal and industrial, fish, spills, or flood control.

Table 4-1
Summary of Predicted Changes in Flow for the Tuolumne and Merced Rivers and Impacts to New Melones Reservoir from A 71-Year Hydrologic Model Analysis of VAMP Conditions

Proposed Action

All of the up to 47,000 acre-feet of supplemental water would come from carry-over storage. Water deliveries provided under the existing VAMP operations would not change. No impact to water deliveries would occur in the MID, TID, and Merced ID service areas.

4.2.3.2 Carry-over Storage in San Joaquin Basin Reservoirs

Carry-over storage is the amount of water retained in a reservoir at the end of September of each water year. The amount of water dedicated to carry-over storage is balanced against the amount needed to meet immediate delivery needs, hydropower generation needs, and instream flow requirements of a project, according to operation rules that differ for each reservoir (State Board 1998).

To determine the changes that would occur due to implementing the Project alternatives on carry-over storage, September end-of-month storage for the Proposed Project are compared to the No Action Alternative.

No Action

The No Action Alternative represents existing conditions. There would be no change in carry-over storage in the two reservoirs that are the source of supply, and no changes would take place in New Melones Reservoir.

Proposed Action

Implementing the Proposed Action would affect carry-over storage in the reservoirs within the Project Area. These changes are described below.

Tuolumne River. During years when supplemental water is required (either April or May), an increase in releases to the lower Tuolumne River occurs and a corresponding decrease in New Don Pedro Reservoir storage results during the pulse flow. In most instances the additional water released is recovered in storage in the following year by a reduction in releases that would otherwise be in excess of minimum Tuolumne River flow requirements. In two instances (e.g., the May 1946 and April 1959 supplemental releases), recovery does not occur for several years.

The changes in carry-over storage in New Don Pedro Reservoir for the April and May releases are shown on Figure 4-1. There would be a decrease in carry-over storage in New Don Pedro Reservoir in about 18 to 20 percent of the 71 years simulated. When supplemental water is needed for April releases, the maximum change in carry-over storage would be a decrease of less than 35,000 acre-feet. For May releases, the maximum change would be a decrease of 45,000 acre-feet. Figure 4-2 shows the same results but as a percentage of the storage under the No Action scenario. The worst case during the 71-year simulation period was a 5 percent decrease that occurred only once. For most years when supplemental water is needed, the decrease in carry-over storage would be less than 3 percent. The impacts to carry-over storage are considered less than significant, given that they are expected to be relatively minor, infrequent, and short term in duration.

Merced River. During the years when supplemental water is required, an increase in releases to the lower Merced River occurs and a corresponding decrease in Lake McClure/New Exchequer Reservoir storage results. In one modeled circumstance (May 1970 and May 1971 supplemental water), a reduction in storage accumulated due to the sequential   provision of supplemental water. In most instances the additional water released would be recovered in storage in the following year by a reduction in releases that would otherwise be in excess of minimum Merced River flow requirements. In two instances (e.g., 1959 and 1971 April supplemental release), recovery did not occur until one or more years later.

The change in carry-over storage in Lake McClure for the April and May releases is shown on Figure 4-3. There would be a decrease in carry-over storage in about 17 to 20 percent of the years simulated. When supplemental water would be needed, for April releases, the maximum change in carry-over storage would be a decrease of less than 35,000 acre-feet. For May the maximum change would be a decrease of about 50,000 acre-feet. Figure 4-4 shows the same results but as a percentage of the storage under the No Action scenario. The worst case during the 71-year simulation period was a 17 percent decrease, which occurred only once in 1971. In about 2 to 3 percent of the years, there would be a decrease in carry-over storage of greater than 10 percent. For most years when supplemental water is needed, the decrease in carry-over storage would be less than 10 percent. The impacts to carry-over storage would be less than significant, given that they are expected to be relatively minor, infrequent, and short term in duration.

4.2.3.3 Changes in Flow and Water Quality

Table 4-2 shows the years where supplemental Spring Pulse Flows above 110,000 acre-feet were needed and the amount of supplemental water required. Supplemental flows were necessary in approximately 10 percent of the years during the modeled period 1922 through 1992.

The above amounts of supplemental water were modeled as coming from either the Tuolumne River or from the Merced River (the Proposed Action). Results of that modeling in comparison to No Action are summarized below.

Table 4-2
VAMP Supplemental Water (1,000 acre-feet)

Click Image to enlarge

Tuolumne River Providing Supplemental Water

For the April supplemental flow setting, the change in flow regime within the Tuolumne River system does not affect operations of the Merced River or the Stanislaus River. Changes within the Tuolumne River occur during periods when Stanislaus River operations are not controlled by Vernalis flow or quality objectives, and a change of Tuolumne River operations outside of the month of the VAMP (April-May) would not affect Merced River operations.

Identical to the April supplemental flow setting, the May setting for Tuolumne River providing the supplemental flow would not affect Merced River operations. However, for the May supplemental water setting, the hydrologic analysis (Appendix A) indicates that Stanislaus River operations may be affected by the recovery of New Don Pedro Reservoir storage in one instance (1985) out of the 8 years requiring supplemental flow in May, which is within the 71 years of modeled sequential hydrology. In this instance, the reduction in release to the Tuolumne River during April 1985 (for recovery of the May 1984 supplemental water) resulted in an increased release from New Melones for water quality objectives at Vernalis. In other words, the Proposed Action could result in a short-term need to increase releases from New Melones Reservoir to meet water quality objectives at Vernalis. The effect on the river and to New Melones operations would be less than significant.

The modeling results represent that the May 1984 supplemental water release may affect water quality in 1985 at Vernalis. Due to the amount of water that is available for water quality releases in the modeled year of 1985, under the New Melones Interim Plan of Operation, the April 1985 water quality release described above came at the expense of being able to fully meet water quality objectives in the following July 1985. Sufficient water exists in New Melones Reservoir to fully comply with the water quality objective. Based on the 71-year hydrologic sequence, this impact would be highly unlikely to occur and is less than significant. Should it occur, Reclamation has the capability to mitigate the potential impact with New Melones Operations.

Merced River Providing Supplemental Water

For the April or May supplemental flow setting, the change in flow regime within the Merced River system would not affect operations of the Tuolumne River. Also, for the May supplemental flow setting, changes within the modeled Merced River flow regime would occur during periods when Stanislaus River operations are not controlled by Vernalis flow or quality objectives; therefore, no changes to the Stanislaus River operation would occur. There would be no impact to water quality from the flow changes.

For the April supplemental water setting, modeling indicates that Stanislaus River operations may be affected by the recovery of Lake McClure/New Exchequer Reservoir storage resulting from the modeled year 1971 supplemental release. In this instance, the supplemental release in the modeled year 1971 resulted in a reduction in release to the Merced River during August 1973. This reduction in summertime discretionary release from Lake McClure resulted in an increased release from New Melones for water quality objectives at Vernalis. The impact occurred only once during the 71-year period. The probability that this impact would occur during the 2001-2010 period is very low. This increased release from New Melones in modeled year 1973 resulted in a minor reduction in spill to the Stanislaus River in March 1975. Any changes to New Melones operations due to the Proposed Action are expected to be infrequent, minor, and short term in duration. Consequently, the impact is less than significant.

4.2.4 Impact Summary and Mitigation of Impacts

4.2.4.1 Water Deliveries

Proposed Action

• Tuolumne River: There would be no changes in water deliveries relative to the No Action scenario, so there is no impact.
• Merced River: There would be no changes in water deliveries relative to the No Action scenario, so there is no impact.

4.2.4.2 Water Storage

Proposed Action

• Tuolumne River: Implementation of the Proposed Action could result in reduced carry-over storage in Don Pedro Reservoir between 15 and 20 percent of the 71 years analyzed. However, the decrease in carry-over storage would generally be less than 3 percent and would always be less than 10 percent. This would be a less-than-significant impact, and no mitigation is required.

• Merced River: Implementation of the Proposed Action could result in reduced carry-over storage in Lake McClure greater than 10 percent in about 2 to 3 percent of the 71-year period. For most of the period when supplemental water is needed, the decrease in carry-over storage would be less than 10 percent. The impact would be less than significant, and no mitigation is required.

4.2.4.3 Water Quality

Proposed Action

• Tuolumne River: Maintaining compliance with the water quality objective at Vernalis was impacted in only one instance (July 1985) due to release of the supplemental water (in May 1984) during the 71-year simulation period. In the context of a hydrologic sequence, the probability of this event occurring over the 2001-2010 period is very small, and it is considered less than significant.

• Merced River: In one instance, the supplemental water release in April 1971 resulted in an increased release from New Melones Reservoir in August 1973 in order to meet the water quality objective at Vernalis. This release resulted in a minor reduction in spill to the Stanislaus River. Any changes to New Melones operations due to the Proposed Action are considered less than significant.