San Francisco Estuary and Watershed Science

The goal of this paper is to analyze storage projects constructed and planned in California since 1980, in contrast with storage constructed before that date. As a result of California’s highly variable climate, storage is an essential tool for agricultural and urban water users. Today, the state regulates approximately 1,250 reservoirs, with a combined storage of 42 million acre-feet. Federal agencies regulate approximately 200 additional reservoirs. The vast majority of this surface storage was constructed before 1978, when New Melones Dam, the last large on-stream water supply reservoir in California, was completed. The role of storage in meeting future needs remains a high-profile issue in the California water debate. For example, funding for new storage was the largest item in Proposition 1, the most recent water bond voters approved. This analysis included a review of existing literature, such as the California Department of Water Resources Division of Dam Safety database, California Water Commission documents about new storage proposals, water agency documents, and interviews with water agency staff and others. Water managers face dramatically different conditions today, in comparison to conditions before 1980. These conditions have led to new approaches to water storage that represent a dramatic departure from past storage projects. During the past 37 years, a wide range of new water storage strategies have been planned and implemented. These facilities have created a combined new storage capacity greater than that of Lake Shasta, California’s largest reservoir. These new storage strategies suggest the need to revisit the fundamental definition of water storage. With limited potential for new storage drawing from the state’s rivers, California must choose storage projects wisely. By learning from successful strategies in recent decades, decision-makers can make better storage investment decisions to help reverse declines in ecosystem health and improve water supply reliability.

In light of desired implementation of subsidence mitigation practices on Delta islands and the need for evaluation tools, we developed groundwater-flow and solute-transport models and attempted to answer the following questions.

1. How do the groundwater-flow and drainage systems interact to influence island drainage volumes and drain dissolved organic carbon (DOC) concentrations and loads?

2. How will future subsidence affect drainage volumes, DOC loads, and seepage onto islands?

3. How will land-use changes to mitigate subsidence affect seepage, drain flow, and DOC loads?

4. How can seepage and water-quality effects from drainage, restoration, and rice cultivation on Delta islands be minimized?

We used hydrologic and geochemical data and modeling to answer these questions. Subsurface processes dominate subsided Delta island hydrology. Seepage and siphoned irrigation water recharge groundwater, which flows to drains. Drainage water that contains DOC derived from oxidation of organic soils is discharged to adjacent channels. We analyzed the effects of subsidence mitigation by simulating mosaics of rice and palustrine wetlands with varying hydrologic management on a representative subsided island (Twitchell Island). These alternative land uses reduce seepage onto islands and thus contribute to increased levee stability. However, most scenarios resulted in increased drain flow and DOC loads. Reducing drain flow is essential to reducing DOC loads relative to the business-as-usual scenario and can be accomplished through hydrologic controls that reduce drain flow on the islands.

For nearly 50 years, the California Department of Fish and Wildlife has used a midwater trawl to intensively monitor fish populations in the San Francisco Estuary during the fall, sampling over 100 locations each month. The data collected have been useful for calculating indices of fish abundance and for detecting and documenting the decline of the endangered fish species delta smelt (Hypomesus transpacificus). However, efforts to calculate estimates of absolute abundance have been hampered by the lack of information on gear efficiency, in particular questions about contact selectivity and the effect of tow method on catches. To answer these questions we conducted a study that used a covered cod end on a net towed either near the surface, referred to as a surface tow, or throughout the water column, referred to as an oblique tow. A contact selectivity model was fit to estimate the probability that a delta smelt that has come into contact with the net is retained in the cod end of the net conditional on its body length. Full retention of delta smelt was found to occur around 60 mm fork length. Delta smelt catch densities for the surface tows were an order of magnitude greater than densities in the oblique tows, suggesting a surface orientation at the sub-adult life stage. These results represent an important step in being able to calculate absolute abundance estimates of the delta smelt population size using decades’ worth of monitoring data.

https://doi.org/10.15447/sfews.2017v15iss4art4

https://doi.org/10.15447/sfews.2017v15iss4art5