Geomorphic Changes Following Channel Modifications In The St. Francis River

The St. Francis River basin covers approximately 8,400 sq. miles in southeastern Missouri and northeastern Arkansas. Local interests and the USACE have completed significant flood control and drainage improvements in the basin, including flow diversion of the Castor River, Wappapello Dam, a leveed floodway, channelization and meander cutoffs. The flow regime and sediment transport capacity of the St. Francis River has been significantly altered as a result of these anthropogenic changes. Portions of the Lower St. Francis River within the leveed floodway are heavily aggradational and has caused tree die-offs, debris jams, and standing water during low-flow conditions. The Memphis District USACE constructed a 5.5-mile drainage channel below Arkansas Highway 90 to provide drainage for the year-round ponded water on the floodway levees. The channel was designed to be cleaned out on a regular maintenance schedule every 5-years and is a difficult task due to frequent high water in the reach. The construction of the channel cleanout typically takes 2-3 years and by the end of the construction period, the channel typically fills back in. The primary objective of this study was to take a comprehensive look at in the 105-mi reach of the St. Francis River below Wappapello Dam to identify sources of sediment to the below Hwy 90 reach. Several analyses of available gage, survey, and LiDAR data, along with historical research, have given an understanding of geomorphic trends and overall river stability. Types of analysis used to determine stage trends included: yearly low stage plots, stage-duration curves, specific gage analysis, water surface slopes, and stream power changes. The results from all analyses were synthesized to develop an overall assessment of the study reach. Aerial imagery analysis showed a 28-mile reduction in channel length for the study area between 1966 and 1975. This reduced the sinuosity of the river and triggered geomorphic change. Immediately following channelization, dramatic decreasing trends in stage were observed for gage locations upstream of the channelization, while the lower reaches began to aggrade. Slopes and stream power were significantly increased for the portion of the study area where channelization had occurred and showed a decreasing trend for the aggradational reach. Concurrently, this effort included the collection of 151 bed samples and 137 bank samples to help characterize the bed sediment and streambank soils within the study reach to develop a data dictionary for future sediment budget development. Results show the St. Francis River is a poorly-sorted sand-bed river overlain by 10 to 20 ft. of silts and clays along the banks. Channel surveys and LiDAR data were also used to compare changes to the channel profiles and identify potential hardpoints or other grade control features.