7820 - Laserfiche WebLink

Home Browse Search

Figure 9. Nursery habitat mapped in September 1992 (41 m3/s) and September 1993 (49 m3/s). Yellow denotes nursery habitat, stippled areas are exposed sand. Letters denote types of habitat (see text) . Figure 10. This detailed map shows the topography of the alternate bar in the upstream part of the study reach in August 1993. Small arrows on the topographic map are bedform migration directions determined by analysis of shallow pits dug in the bar surface. These data will be used in analysis of accuracy of modeled flow velocity and transport directions. Dark blue areas are nursery habitat available at a discharge of about 40 m3/s; lighter blue areas shown on the overlay are available nursery habitat at a discharge of about 125 m3/s. The diverse topography of the upper bar surface, formed by irregularities of late recessional scour channels and migrating bar slipfaces is such that there is a greater area of stagnant water at higher stage than at those stages when the entire bar is subaerially exposed. Even at the higher mapped discharge, most of the downstream scour channel is disconnected from the main channel and nursery fish in this area can only return to the main channel if flow subsequently increases. Deeper areas of stagnant flow provide habitat over a wider range of discharges. Topographic map and bedform analysis completed by Reed Krider, Geology Department, Carleton College, as part of his undergraduate senior thesis. Figure 11. Area of available nursery habitat and available high-quality nursery habitat are shown in relation to discharge. Habitat suitability curves were plotted by eye, based on the assumption that available in-channel habitat must be zero at zero discharge and at discharges that completely inundate bar surfaces. For the same discharge, the area of available habitat was about 30 percent less in 1993 than in 1992, but the decrease in the area of high-quality habitat was much less. Sufficient data was available for the 1992-93 period to determine that maximum available habitat exists at some discharge between 40 and 100 m3/s, perhaps at about 70 m3/s. We will not know the shape of the 1993-94 curve until data is collected in spring 1994. Figure 12. Although the shape of habitat suitability curves differs from year-to-year, our data suggests that data from all years can be fit to one suitability curve if discharge is converted to a new parameter -- relative water stage in relation to average elevation of bar surfaces. Consistent with the data presented in Figure 11, Figure 13 shows that maximum available habitat exists at some critical elevation below the average elevation of the highest part of the bar surface. This elevation is probably equivalent to the average base of scour channels and dune scour pits. Additional data will be collected to test the hypothesis generated by this figure and to determine more precisely the shape of this curve. Additional data will also be collected to determine if the curve for high quality habitat actually peaks at the same discharge as the curve for all habitat. Figures 11 and 12 show that base flows in the Green River should not be based on any absolute value of discharge. Instead, baseflows (if intended to maximize available nursery habitat) should be established in relation to the average elevation of bar surfaces constructed by each spring's runoff. Modeling strategies must be used to estimate the magnitude and duration of peak flows that build bars to specific elevations, and empirically-derived habitat suitability curves (if presented in terms of relative water stage) can be used to quantify appropriate baseflows.