Abstract

As part of a collaborative study of the cumulative impacts on stream and riparian ecology of water and channel management practices in the lower Platte River, Nebraska, this report describes a study by the U.S. Geological Survey in cooperation with the Lower Platte South Natural Resources District that summarizes: (1) temporal differences in distribution of streamflow depth, velocity, and microhabitats among five discrete 11-water-year periods 1934–44, 1951–61, 1966–76, 1985–95, and 1996–2006, and (2) the effects of bridge proximity on distribution of streamflow depth, velocity, and microhabitat of the Platte River when cross sections were measured at a similar discharge. The scope of the study included the four presently (2008) active streamflow-gaging stations located near bridges over the lower Platte River at North Bend, near Leshara, near Ashland, and at Louisville, Nebraska, and the most downstream streamflow-gaging station within the central Platte River segment near Duncan, Nebraska.

Generally, in cases where temporal differences in streamflow depth and velocity were evident, at least one of the water-year periods from 1934 through 1995 had deeper streamflow than the recent water-year period (1996–2006). Temporal differences in distributions of streamflow depth were not strongly associated with differences in either climatic conditions or the maximum peak flow that occurred prior to the latest discharge measurement during each period. The relative cross-sectional area of most hydraulic niches did not differ among the water-year periods. Part of this apparent uniformity likely was an artifact of the broad microhabitat classification used for this study. In cases where temporal differences in relative cross-sectional area of hydraulic niches were evidenced, the differences occurred during high- and low-flow conditions, not during median flow conditions. The temporal differences in relative cross-sectional area were found more frequently for hydraulic niches defined by moderate and fast velocities than for hydraulic niches defined by slow velocities. Generally, any significant increase or decrease in the relative cross-sectional areas of hydraulic niches during the water-year periods from 1934 through 1995 had disappeared during the most recent water-year period, 1996–2006.

Deep-Swift niche was the predominant hydraulic niche for all near-bridge sites on the lower Platte River for high- and median-flow conditions. The Deep-Swift niche also was the predominant niche for the near-bridge sites near Ashland and at Louisville for low-flow conditions; for the near-bridge sites at North Bend and near Leshara, streamflow cross-sectional areas during low-flow conditions were shared among the Shallow-Moderate, Intermediate-Moderate, Intermediate-Swift, and Deep-Swift hydraulic niches. For the near-bridge site near Duncan, the site farthest downstream in the central Platte River system, the Deep-Swift hydraulic niche was predominant only during high-flow conditions; during median- and low-flow conditions the relative cross-sectional area was shared among the Shallow-Slow, Shallow-Moderate, Intermediate-Moderate, and Intermediate-Swift hydraulic niches.

Significant temporal differences in the relative cross-sectional area of the Deep-Swift hydraulic niche were found for sites near the two farthest downstream bridges near Ashland and at Louisville, but only for low-flow conditions. The Deep-Swift microhabitat was of special interest because it is the preferred hydraulic habitat during the adult life of the endangered pallid sturgeon (Scaphirhynchus albus). Temporal differences in relative cross-sectional areas of the Glide low-flow geomorphic microhabitat that contained the Deep-Swift hydraulic niche also indicated that relative cross-sectional areas of the Glide during the 1951–61 and 1996–2006 water-year periods were lower than during the 1966–76 period. The temporal differences indicated that any significant temporal change in relative cross-sectional area of the Deep-Swift hydraulic niche, and the Glide and Race microhabitats near the two farthest downstream bridges (near Ashland and at Louisville) since the 1951–61 water-year period had nearly disappeared during the most recent water-year period (1996–2006). It may indicate no evidence of net reduction in the relative cross-sectional area of either the Deep-Swift or the Glide and Race habitats near the two farthest downstream bridges (near Ashland and at Louisville).

The effects of bridge proximity on streamflow depth, velocity, and microhabitats were evaluated for median- and low-flow conditions; high-flow conditions were not evaluated because no cross-sectional measurement was made for beyond-bridge sites. Streamflows for median-flow conditions were deeper for the near-bridge sites than the beyond-bridge sites at North Bend and near Ashland for the 70th or higher percentiles of the area cumulative frequency distribution. Generally, for low-flow conditions, bridge proximity had no effects on the streamflow depths for sites at North Bend and near Leshara. The effect of bridge proximity on streamflow velocities was either absent or inconclusive. Bridge proximity affected four of the nine hydraulic niches. The relative cross-sectional areas of the Deep-Moderate and Intermediate-Slow hydraulic niches were larger (at Louisville) or similar (at North Bend) for the near-bridge site than for the beyond-bridge site during median-flow conditions. The relative cross-sectional area of the Deep-Moderate hydraulic niche (at North Bend) and the Shallow-Moderate (at Louisville) was larger for the beyond-bridge site than the near-bridge site. The near-bridge and beyond-bridge sites did not differ in relative cross-sectional area of the Deep-Swift hydraulic niche for median-flow conditions. Historical cross-sectional measurements made near near-bridge sites can be used as a primary data set in hydraulic-habitat study, before embarking on a more spatially intensive but costly program of streamflow-depth and -velocity data collection.

First posted December 18, 2008