Laserfiche WebLink
Sediment Trap Data: Streambed sediment traps were installed in riffle and run habitats to <br />monitor the movement of fine sediment (broadly defined) on the receding limb of the hydrograph <br />when Colorado Pikeminnow normally spawn. The primary objectives of the trap measurements <br />were to determine the sizes of sediment in transport at that time, and to a lesser extent, to provide <br />qualitative information on transport rates. If one of the goals of coordinated reservoir releases is <br />to flush fine sediment from the bed to improve micro-habitats , then it is reasonable to consider <br />how long the benefits of a flushing flow may last. <br />The figures on the following page summarize the results from the trap measurements. <br />Hydrographs for the period of snowmelt runoff are shown for each of the four years in which the <br />traps were used, 1998-2001. The vertical lines on the hydrographs indicate specific dates that <br />the trap samples were taken. The figures to the right of the hydrographs show the grain size <br />distribution of the sediment taken from the traps; these do not include the first sample of the <br />year, which would include sediment collected any time during the previous 9-10 months. For <br />comparison, these figures also show the grain size distribution of the bed material (red lines), as <br />determined from three bulk samples of the subsurface sediment in the 15 mile reach. <br />The first point to note in these figures is that the grain size of the sediment caught in the <br />traps is much finer than the sediment sampled from the bed. The median grain size of the <br />subsurface sediment, Dsos, is about 30 mm (medium gravel), whereas the median grain size of <br />the trapped sediment is typically between 0.1 and 0.5 mm (fine-medium sand). About 10% of <br />the subsurface sediment falls in that size range. The sediment caught in the traps is found in <br />appreciable quantities in the bed, therefore, it represents a component of what Einstein [19501 <br />termed the bed material load (the other component- termed wash load- is sediment that is not <br />found in appreciable quantities in the bed; silt and clay fall into that category in this case). <br />42