Channel Morphology and Bed-Sediment Characteristics Before and After Riparian Vegetation Clearing in the Cottonwood Ranch, Platte River, Nebraska, Water Years 2001-2004

By Paul J. Kinzel, Jonathan M. Nelson, and Ashley K. Heckman

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Cover photograph: Oblique aerial photograph of the Platte River, Cottonwood Ranch study reach (center), taken March 26, 2003 (top of photograph is west/upstream). Photograph courtesy of Lawrence R. Davis, Davis Aviation.

The citation for this report, in USGS format, is as follows:
Kinzel, P.J., Nelson, J.M., and Heckman, A.K., 2006, Channel morphology and bed-sediment characteristics before and after riparian vegetation clearing in the Cottonwood Ranch, Platte River, Nebraska, water years 2001-2004: U.S. Geological Survey Scientific Investigations Report 2005-5285, 25 p.

Abstract

Riparian areas along a reach of Platte River passing through Nebraska Public Power District's Cottonwood Ranch Property were modified during 2002 to 2004 to enhance in-channel habitats for endangered and threatened avian species. A component of this alteration involved the removal of riparian vegetation from riverbanks and islands to provide roosting habitat for the endangered whooping crane and to provide nesting and foraging habitat for the endangered least tern and threatened piping plover. It was hypothesized that the removal of riparian vegetation could have the effect of stimulating channel widening in this reach by increasing the potential of these surfaces to erode under natural fluvial action. It also was hypothesized that as a direct or indirect consequence of the alterations, a local increase in sediment supply also might occur, potentially resulting in geomorphic change downstream and possibly initiating negative third-party effects. The cumulative effects of the management activities on the channel morphology and sediment transport in this reach were monitored during water years 2001-2004 by measuring transect elevation profiles and bed-sediment-size gradations upstream, within, and downstream from the managed area before and after the development activities.

An analysis of variance (ANOVA) was performed to determine if the geomorphic variables measured before and after the development activities were significantly different. Although statistically significant differences were detected in some of the variables, increases in mean bed elevation did not occur in a greater percentage of the monitoring sections measured downstream compared to upstream from the management activities. This result suggests that the management activities did not have a substantial effect on the downstream river channel morphology and sediment transport. However, it is important to place these short-term and site-specific results in the context that river flows following the management activities were at historical low rates, and therefore the potential to affect and the opportunity to detect possible geomorphic change within and downstream from the managed reach were limited.

Table of Contents

Abstract

Introduction

Purpose and Scope

Acknowledgments

Study Area

Geomorphology

Hydrology

Management Activities

Methods

Streamflow

Geomorphic Monitoring

Channel Morphology

Sampling of Bed and Bank Sediment

Ground Photography

Aerial Photography

Statistical Analysis

Channel Morphology

Bed-Sediment Characteristics

Additional Analysis

Stage Trend

Trends in Mean Bed Elevation from Channel Surveys

Streamflow

Summary and Conclusions

References Cited

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