Flash Floods, Sediment Transport, and the Geomorphic Transformation of Moenkopi Wash, AZ

Abstract:

Fine sediment is a crucial resource to the Colorado River ecosystem in Grand Canyon National Park, AZ. Much of the fine sediment is supplied during flash floods on the Little Colorado River (LCR) and its tributary, Moenkopi Wash (MW). Concentrations of suspended-sediment during flash floods on MW are often greater than 200,000 mg/L, and have exceeded 490,000 mg/L. Historically, the highest concentrations of suspended-sand measured in the LCR have been the result of flooding on MW.

Over the last 65 years, widespread geomorphic change on MW has occurred that has likely altered the amount of fine sediment supplied to the LCR. Geomorphic changes include channel-bed aggradation, channel narrowing, increases in sinuosity, and non-native vegetation colonization of the banks and floodplains. Channel-bed aggradation has exceeded 2m, and the channel has narrowed by more than 66% in some areas. In these areas, the combination of aggradation and narrowing has resulted in the loss of cross-section area by nearly 90%. The direct causes of these changes are unclear and may include changes in stream flow and sediment supply caused by human activities and/or climate, and the increased sediment trapping efficiency of vegetation. However, the causes are likely a complex combination of all of the above. Although stream gages have existed on Moenkopi Wash since 1926, frequent changes in gaging locations combined with substantial flood attenuation during flash floods make it difficult to compile a long-term flow record at a single location. Therefore, we are using a kinematic-wave model to route floods between the historic gaging locations in order to construct a long-term flow record near the mouth of Moenkopi Wash. Analyses of the long-term flow record, combined with analyses of suspended-sediment transport are used to evaluate how the geomorphic transformation of Moenkopi Wash has affected sediment delivery to the Little Colorado and the Colorado Rivers.