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14000 12000 10000 i 8000 .. '" ~ .c u .. 6000 2i 4000 2000 IOLate-summer/tall Basetlows '~L : l:JWinter Basetlows I l:JRising Limb Iiiil Peak Flows I I IiiIFalling Limb . ~ I - - - l o 1-0ct 1-Nov '-Oec 1-Jan 1~Feb '-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep Date Figure ES-2. Mean annual hydrograph for the Colorado River at Palisade gage showing the subdivisions of the hydro graph discussed in the text. The bulk of the annual sediment load is transported during the high-flow period when both the suspended load and bedload is transported (Pitlick et aI. 1999) (Figure ES-3). The peak flows are, therefore, both morphologically and biologically important (Power, 2001). Pitlick et aI. (1999) computed a reach-averaged critical discharge of about 10,000 cfs, and a discharge of about 22,000 cfs for general bed mobilization. Because of the effects of summer thunderstorms, suspended-sediment concentrations can be higher during the base flow periods than peak flow periods. The quantity of sand, silt, and clay that is transported during the low-flow periods is relatively small, is dominated by silt and clays, and is morphologically unimportant but biologically very important. However, because of the stochastic nature of the thunderstorm events that generate the fine sediment, the sediments are not well represented in the USGS Cameo gage record that is based on fixed-interval sampling. The physical system can be divided into two somewhat separate, but interconnected, response regimes, framework responses, and transient responses (Figure ES-4). Both have the ability to affect the biological productivity of the system by producing physical disturbances that affect both periphyton and macroinvertebrate trophic levels (Hildrew and, Townsend 1987; Biggset aI., 1998). Framework responses are related to mobilization of the gravels and cobbles that form the bed of the river, and provide the habitat for the periphyton and macro invertebrates (Biggs et aI., 2001). In the context of the Clifton site, incipient conditions for the bed armor layer (Dso about 80 mm) occur at snowmelt runoff discharges that exceed approximately 4,800 cfs (less than a 1.. year recurrence interval) in the riffles and about 13,000 cfs (about 1.6-year recurrence interval) in the runs. Mobilization and transport of the run and riffle sediments at discharges greater than 20,000 cfs (2.8-year recurrence interval) create a substantial disturbance to and reset of both the Draft Final Report - 15 Mile Reach Studies Miller Ecological Consultants, Inc. Page ES-6 October 24, 2003