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Show Major Findings 7 headwaters) to 0.49 mg/ L below Cutler Reservoir ( near the mouth) ( fig. 9). Concentrations of phosphorus in the contributing tributaries averaged about 0.46 mg/ L and were associated primarily with natural phosphorus deposits, manure from animal feeding operations, fertilizers used on farm fields, and wastewater- treatment discharge. In contrast, concentrations of total phosphorus in the Bear River in July/ August 2001, when large amounts of water were diverted for irrigation, were highly variable, especially above and below dams. For example, concentrations of total phosphorus generally increased below reservoirs upstream from Bear Lake ( possibly due to low water levels in the reservoirs, which allowed resuspension of particulate phosphorus) and decreased below most reservoirs downstream from Bear Lake ( due to deposition of particulate phosphorus in the reservoirs). During this period, tributaries generally were diverted upstream from their confluence with the Bear River and contributed little flow or contaminants to the Bear River. Similar patterns in water quality above and below diversions also were noted in other streams ( fig. 10). In general, streams above diversions generally drain undeveloped basins and water is of good quality. Below diversions, streamflow consists mainly of agricultural and urban runoff, ground- water inflow, and inflow from irrigation canals and wastewater- treatment facilities, which can contain elevated levels of 3.51 1 700 Kays Baer Kays Baer Creek Creek Creek Creek Figure 10. Good quality water from undeveloped basins is diverted at canyon mouths. Agricultural and urban runoff replaces this water, and the quality of water in the stream is degraded. suspended sediment, nutrients, organic pollutants, and dissolved solids. For example, the concentration of dissolved solids above diversions on Baer Creek, a small stream in the Weber River Basin between Ogden and Salt Lake City, was less than 100 mg/ L, whereas the concentration below diversions was greater than 500 mg/ L. Fish communities in urban streams respond to changes in water quantity Water management infrastructure and operation affect fish community composition and the ability of urban streams to support fish. These factors are particularly important in moderate- sized watersheds, such as Little Cottonwood, Big Cottonwood, and Mill Creeks in Salt Lake Valley, Utah. These watersheds are similar in size and environmental setting but have very different fish communities, in large part due to withdrawals of water. Water is withdrawn from Little and Big Cottonwood Creeks before they flow into urban areas, sometimes resulting in complete dewatering of the channel below the withdrawal point in the summer, when natural flows are low ( fig. 11). In Mill Creek, smaller diversions remove water, but there is not a large withdrawal that completely dewa- ters the channel. Water is restored to all three stream channels downstream from diversions through ground- water inflow and additions through canal systems. The additions to Big Cottonwood Creek are more substantial than those to Little Figure 11. Segments of many urban streams in the Study Unit, such as the one shown here on Little Cottonwood Creek, are completely or nearly dewatered in the summer when natural flows are low and water use is high. Cottonwood Creek, and as a result, flow in Big Cottonwood Creek is more nearly continuous and reliable in the summer. Fish communities in these streams reflect the differences in flow. For example, in Little Cottonwood Creek, which had the largest diversions and least flow, the fish community consisted of mountain sucker, speckled dace, and fathead minnows. These small- bodied fish are able to survive in low- flow, shallower environments. In Mill Creek, which had the least diversion and most flow, the dominant fish collected was brown trout, a larger species. The fish community of Big Cottonwood Creek was a mix of mountain sucker ( the dominant fish species in Little Cottonwood Creek), brown trout, and a variety of other species consisting primarily of sunfishes, minnows, and catfish ( fig. 12). Alterations of natural flow in stream channels can adversely affect water quality and the availability and quality of fish habitat. Consequently, Little Cottonwood Creek Cottonwood Creek Mill Creek - 1 1 1 1 1 - • • 1 1 1 1 • « 1 II 1 • • • •- 1 1 1 1 1 1 1 1 1 1 1 1 1 Dominant fish 5 10 15 20 25 12 STREAMFLOW, IN CUBIC FEET PER SECOND 16 18 20 0.5 0.10 0.15 0.20 0.25 0.30 AVERAGE SUMMER DISSOLVED ORGANIC TEMPERATURE, IN NITROGEN, IN MILLI- DEGREES CELSIUS GRAMS PER LITER Figure 12. Among three sampled urban streams, the two with lower streamflow ( and greater withdrawals) had warmer water temperatures and poorer water quality ( shown by higher concentrations of dissolved organic nitrogen) and were dominated by smaller fish species. |