Trends in Hydrology and Salinity in Suisun Bay and the Western Delta
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Trends in Hydrology and Salinity in Suisun Bay and the Western Delta Draft Version 1.2 June 2007 DRAFT Trends in Hydrology and Salinity in Suisun Bay and the Western Delta Draft Version 1.2 Executive Summary........................................................................................................................ 2 Objective..................................................................................................................................... 2 Approach..................................................................................................................................... 2 Conclusions................................................................................................................................. 3 Report Structure.......................................................................................................................... 7 1. Introduction............................................................................................................................. 8 1.1. Objectives of this review ................................................................................................. 8 1.2. Salinity Units................................................................................................................... 9 1.3. Temporal and Spatial Variability................................................................................... 13 1.4. Report Structure............................................................................................................. 14 2. Factors Influencing Salinity Intrusion in the Delta............................................................... 16 2.1. Climatic variability........................................................................................................ 17 2.2. Physical changes to the Delta and Central Valley ......................................................... 19 2.3. Flow Management Regimes .......................................................................................... 21 3. Historical Context – The last 3,000 years............................................................................. 28 3.1. Reconstructed Salinity in Northwestern Suisun Marsh (~1000 B.C – 2000 A.D) ....... 28 3.2. Reconstructed Unimpaired Flow in the Sacramento Valley (900 A.D. – 1976 A.D.)... 29 4. Qualitative Salinity Observations (late 1700s – early 1900s).............................................. 30 4.1. Observations from Early Explorers ............................................................................... 30 4.2. Observations from early settlers in the Western Delta .................................................. 33 5. Quantitative Salinity Observations (early 1900s – present) ................................................ 39 5.1. Fluctuation or Movement of the Spatial Salinity Distribution....................................... 39 5.2. Trends in Salinity at Specific Locations ........................................................................ 52 6. Conclusions........................................................................................................................... 62 7. References............................................................................................................................. 67 Supplemental Figures.................................................................................................................... 70 DRAFT: Last modified 6/9/2007 10:51 PM 1 D_report_flow_salt_trends_v1p2.doc DRAFT Executive Summary Objective Prompted by recent discussions that the Sacramento-San Joaquin Delta (Delta) is currently managed as a freshwater system with significantly less fluctuation in salinity than would occur under “natural” conditions, this report examines present-day salinity levels and variability in the context of historical conditions. The following questions are addressed in this report: • Average salinity: How does the average present-day salinity at specific locations in the western Delta and Suisun Bay compare with average salinity at the same locations at different times in history? • Seasonal fluctuations in salinity: How does the seasonal variability in salinity in the present day compare with historical seasonal salinity fluctuations? • Inter-annual fluctuations in salinity: How do maximum and minimum annual salinity intrusion during wet and dry years in the present day compare with historical salinity intrusion under similar hydrologic conditions? • Flow management: How is variability at various timescales altered by the cumulative impact of upstream diversions, reservoir operations, in-Delta diversions, and south of Delta exports? The following questions are not addressed: • What salinity regime should be imposed on the Suisun Bay and Delta? • Is salinity the best metric to evaluate ecosystem dynamics? What other metrics should be considered? Approach Hydrology and salinity in Suisun Bay and the western Delta vary extensively in both space and time. Misinterpretation of the location or time period of a given observation, or comparisons between conditions at widely separated locations can cause confusion. Confusion may be compounded and lead to conflicting conclusions when salinity is discussed in qualitative terms such as “fresh” and “brackish”. To clarify discussions, careful references to quantitative salinity levels, time periods for comparison, and locations in the Delta are made throughout this report. The introduction to this report provides contextual information on the temporal and spatial variability of salinity in Suisun Bay and the western Delta. To establish a timeline DRAFT: Last modified 6/9/2007 10:51 PM 2 D_report_flow_salt_trends_v1p2.doc DRAFT of natural and anthropogenic modifications, Section 2 presents a qualitative discussion of factors that influence salinity intrusion. The balance of this report is a chronological review of salinity observations, referencing paleoclimatic reconstructions (Section 3), qualitative observations (Section 4), and quantitative measurements (Section 5). Conclusions Key findings for each research question are summarized here with reference to more detailed information within this report. • Average salinity On average, the Delta is not significantly fresher today. Paleosalinity analyses and evidence from the early 1900s both suggest that the Delta is actually saltier today than in the early 1900s. Paleoclimatic research provides evidence of salinity variability prior to European influence. Analyses of sediment cores in northwestern Suisun Marsh indicate that the marsh has experienced centennial-scale cycles of fresh and brackish conditions. During the last century, salinity has increased substantially in comparison to the previous 600 years and has been as salty as or saltier than periods as far back as 2,500 years before the present time. (Section 3.1, Page 28) The earliest quantitative measurements of salinity were recorded by the California & Hawaiian Sugar Refining Corporation (C&H) from 1908 to 1929; C&H measured the distance its barges had to travel upstream on the Sacramento and San Joquin rivers to obtain water suitable for their refinery and quantified the salinity of the water. From 1908 through 1917, the farthest monthly averaged distance traveled to obtain suitable water was 30 miles upstream of Crockett (below Jersey Point on the San Joaquin River). By 1920, the quality of water easily obtained by C&H barges had degraded due to increased upstream irrigation diversions, especially for newly introduced rice cultivation, to the point that C&H abandoned the Sacramento and San Joaquin Rivers during the summer and fall, replacing the water supply with an agreement with Marin County. Comparison of the C&H observations for 1908 through 1917 (prior to significant upstream diversions) with recent data indicates that there has been a 7-fold increase in salinity in the western Delta and Suisun Bay. The location of the 50 mg/L chloride (350 µS/cm EC) isohaline observed by C&H from 1908 through 1917 is approximately the same as the location of X2 (2,640 µS/cm EC) in recent years with similar unimpaired hydrology (1995 to 2005). The distance from the C&H plant to 50 mg/L chloride water is greater in the last 10 years than it was in the early 20th century, with the largest increase in the spring. Fresh water (less than 350 µS/cm EC) has been located below the confluence of the Sacramento and San Joaquin Rivers approximately 42% of the time from 1995 to 2005, compared to 72% percent of the DRAFT: Last modified 6/9/2007 10:51 PM 3 D_report_flow_salt_trends_v1p2.doc DRAFT time from 1908 to 1917, indicating that the Delta is now saltier on average for a similar hydrologic period. (Section 5.1.1, Page 39) Long-term monitoring data at fixed locations in the Bay and the Delta started as early as 1920. Previous analyses of trends in Suisun Bay and the western Delta indicate that centennial-scale trends in outflow and salinity are much smaller than seasonal and decadal trends (Enright et al, 2004; Fox, 1987). Salinity at Collinsville (near the confluence of the Sacramento and San Joaquin Rivers) gradually decreased from 1930 to about 1967 but has been increasing from 1967 to present. (Section 5.2, Page 52). • Seasonal fluctuations in salinity Seasonal variability has significantly changed over multiple time periods. Relative to the earliest salinity measurements (1908-1917), currently: • salinity begins to intrude in the mid-winter, instead of the early-summer; • the