Sediment delivery by ungaged tributaries of the Colorado River in Grand Canyon

Abstract

Sediment input to the Colorado River in Grand Canyon, Arizona, is a valuable resource required to sustain both terrestrial and aquatic ecosystems. A total of 768 ungaged tributaries deliver sediment to the river between Glen Canyon Dam and the Grand Wash Cliffs (river miles 15 to 276). The 32 tributaries between the dam and Lee’s Ferry produce only streamflow floods, whereas 736 tributaries in Grand Canyon produce streamflow floods and debris flows. We used three techniques to estimate annual streamflow sediment yield from ungaged tributaries to the Colorado River. For the Glen Canyon and Marble Canyon reaches (river miles -15 to 61.5), respectively, these techniques indicate that 0.065 . 10 6 and 0.610 . 10 6 Mg/yr (0.68 . 10 6 Mg/yr of total sediment) enters the river. This amount is 20 percent of the sediment yield of the Paria River, the only gaged tributary in this reach and a major sediment contributor to the Colorado River. The amount of sand delivered ranges from 0.10 . 10 6 to 0.51 . 10 6 Mg/yr, depending on the sand content of streamflow sediment. Sand delivered in Glen Canyon is notably coarser (D 50 = 0.24 mm) than sand in other reaches (D 50 = 0.15 mm). A relation is given for possible variation of this sediment delivery with climate. Debris flows transport poorly-sorted sediment onto debris fans in the Colorado River. In the pre-dam era, debris fans were completely reworked during Colorado River floods, liberating all fine-grained sediment to the river; in the post-dam river on average only 25 percent of debris-fan volume is reworked, leading to storage of sand in the matrix of debris fans. We develop a sediment-yield model for debris flows that uses a logistic-regression model of debris-flow frequency in Grand Canyon, a regression model of debris-flow volumes, particle-size distributions of intact debris-flow deposits, and debris-fan reworking. On average, debris flows deliver between 0.14 . 10 6 and 0.30 . 10 6 Mg/yr of sediment to debris fans throughout Grand Canyon. Together, streamflow and debris flow deliver nearly 2.8 . 10 6 Mg/yr of sediment to the Colorado River from ungaged tributaries. In the post-dam era of minimal debris-fan reworking, the combined sand delivery rate in Glen and Marble Canyons averages 0.32 . 10 6 Mg/yr, which is 20 percent of the sand delivery of the Paria River and double the 0.17 . 10 6 Mg/yr of sand estimated for this reach in the 1995 environmental impact statement for operation of Glen Canyon Dam.