Sand-storage changes in the Colorado River downstream from the Paria and Little Colorado Rivers, June 1992 to February 1994

Abstract

Sixty-six cross sections on the Colorado River downstream from the Paria and Little Colorado Rivers were monitored from June 1992 to February 1994 to provide data to evaluate the effect of releases from Glen Canyon Dam on channel sand storage and for development of multidimensional flow and sediment-transport models. Most of the network of monumented cross sections was established and first measured June-September 1992 and remeasured four times between then and January-February 1994. Each measurement consisted of 10 passes across the section, and data presented are the mean section and the standard deviation from the mean. Measured depths were converted to bed elevations using water-surface elevations measured or estimated for each reach. A line marked at regular intervals was strung across the river between the section end points and used to provide horizontal position control. A Wilcoxon rank-sum test was applied to the data, and bed-elevation differences between successive measurements that were statistically significant at the 5-percent significance level were identified and used to compute the difference in cross-sectional area from measurement to measurement. This report presents the data from the five measurements made during the reporting period in graphical form and describes the electronic form of the data. Bed-elevation differences and the computed/? value from the Wilcoxon rank-sum test are included in electronic form, and the format of the electronic files is described in the report. INTRODUCTION In the early 1980's, agencies charged with management of the Colorado River in Grand Canyon, white-water rafters, and anglers became concerned that flow releases from Glen Canyon Dam were eroding sandbars that are critical to the riparian system in Grand Canyon National Park (fig.l). Concern about sandbars has focused on potential degradation by unsteady dam releases for power generation. Since 1982, the Bureau of Reclamation has coordinated a comprehensive program of investigations the Glen Canyon Environmental Studies (GCES) to determine the effects of dam releases on the riparian and aquatic resources of the Colorado River downstream from the dam. In 1989, as a part of the GCES, the U.S. Geological Survey (USGS) began a program of field-data collection and model development aimed at the production of a suite of flowand sediment-transport models to monitor sand movement and to predict sediment response to releases. Because of growing concern over the effects of dam releases on riparian resources, restrictions were placed on releases by Congress under the Grand Canyon Protection Act in 1992. The restrictions, called Interim Flow Criteria, set limits on maximum and minimum daily releases and on the rate of increase and decrease of releases. Interim flow criteria were followed during the period of measurements presented in this report, and the daily mean discharge at Lees Ferry, Arizona, was between about 200 and 500 m3/s Abstract 1