MANAGEMENT IMPLICATIONS FOR BEDLOAD MODELS IN THE GRAND CANYON OF THE COLORADO RIVER

On the Colorado River in Grand Canyon National Park, sediment supply may be as important as water discharge in regulating sediment flux. This condition, known as “supply limitation”, precludes stable sediment flux rating curves with discharge. Instead, sediment monitoring programs utilize measurements of acoustic surrogates for suspended sand concentration to construct flux-based sediment budgets. These techniques provide accurate estimates of suspended sand flux, but estimates of total load depend on the assumption that bedload flux is a constant fraction of suspended sand load. Currently, bedload is treated as a constant 5% of suspended sand load based on one set of observations obtained at one site (Rubin 2001).

We test this assumption using a new model for bedload flux (Ashley et al, in prep) designed to reproduce flow strength and supply-limitation effects using quantities that are routinely measured at sediment monitoring stations on the Colorado River. This model indicates that the cumulative bedload flux summed over the existing gage record is approximately 5% of the cumulative suspended sand flux, but that the instantaneous ratio of bedload to suspended load ranges from 1% to 75% during this time. Low bedload fractions occur during floods when the channel is enriched with fine sediment, whereas high bedload fractions occur during fine-depleted base flows. Sediment budgets are sometimes used to evaluate the short-term morphodynamic trends associated with individual or seasonal changes in the dam-regulated flow regime; it is therefore critical to account for bedload variability at this scale. Sediment budgets are also used to assess long-term trends in sediment storage. Though the average bedload fraction over the last decade was approximately 5%, this is not guaranteed to be the case in the future. The long-term average bedload fraction reflects the sediment supply and discharge regime, which both may evolve over time. Dam-regulated water discharge is the primary tool for enacting management decisions in the canyon, and changing land use and climate are likely to impact the rate of tributary sand delivery.