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california Water Today 83 The quality of water for environmental uses varies with the species or eco- system of concern, and water management for human uses has often disturbed the natural conditions in which native species thrive. Artificially high water temperatures in many California streams—resulting from dams, diversions, streamside development, and irrigation—limit spawning and rearing habitat for salmon and other fishes (Chapter 5). Agricultural and urban runoff often adds diverse contaminants to streams, harming aquatic species.6 In the Sacramento– San Joaquin Delta, native species thrive in murky, muddy water, with more vari- able salinity, and the system’s use as a conveyance hub has made it artificially more stable and clearer, favoring invasive species (Moyle and Bennett 2008; Moyle et al. 2010). A general problem in California is that as streams become more altered in flows and water quality, alien fishes, invertebrates, and plants tend to become predominant (Brown and Moyle 2004; Brown and Bauer 2009). On the other hand, treated wastewater provides much of the flow in some sec- tions of the Santa Ana River, and it is of high enough quality to support a diverse fish fauna, including the endangered Santa Ana sucker (Catostomus santaanae) (Brown, Burton, and Belitz 2005). Salinity and other contaminants Local runoff and stream flow accumulate dissolved solids, salts, and nutrients as they flow downstream from pristine upper mountain watersheds. Likewise, aquifer quality varies widely. In some areas, groundwater is so pure that it requires no treatment for direct potable use, whereas in others, salinity and other contaminants necessitate blending or costly wellhead treatment. Statewide, salinity is the most widespread quality concern, both for aquifers and surface flows. Salts come from several sources: They occur naturally in min- erals in some soils (where they are released by precipitation or excess irrigation), and they are also present in mineral-based fertilizers and urban wastewater. The salinity of many streams and aquifers has increased as a result of irrigation and urban water uses. When the rate of salt input exceeds the rate of discharge, salts accumulate in soils, water bodies, and aquifers. Salt accumulation can change conditions for ecosystems, reduce the productivity of soils for agriculture, and increase costs for urban water users (Box 2.2). Salinity problems are greatest in the southern Central Valley and the Salton Sea. High salinity in the lower San Joaquin River from agricultural drainage 6. See Brown (2000) for an illustration relating to the San Joaquin River.