SEDIMENT SOURCES OF THE LEWIS AND CLARK LAKE DELTA, MISSOURI RIVER

The unimpeded growth of the Lewis and Clark Lake delta will continue to impact multiple facets of Lewis and Clark Lake and the adjacent upstream Missouri River segment. Future river and sediment management decisions should be informed by data on sediment sources. In 2015, we collected bedload (sand) and suspended load (silt and clay) to determine sediment sources of the delta. Our measurements reveal that tributaries are an important source of suspended load. The Niobrara River, the largest tributary in the study area, is an important source of sand, whereas the other tributaries are of minor importance because low flow rates limit sand transport. Geochemical analysis of surficial sands from channels and sand bars indicates that roughly half the delta sand is derived from the Niobrara River, and the other half from the Missouri River. In contrast, core samples collected at depth from one sand bar have a close affinity to the Niobrara River. Sediment load estimates from other studies suggest that the Niobrara River is the dominant source of sand to the delta. We interpret our data to indicate that the Niobrara River is the dominant day-to-day source of sand to the delta, while the Missouri River is the dominant source during high flow events. Previous work has suggested that major building phases of the delta are associated with high water events, such as in 1997 and 2011. While the Niobrara River is clearly an important source of sediment to the Lewis and Clark Lake delta, high water events on the Missouri River are geomorphically significant over the long term by scouring and depositing large amounts of sediment from banks, sand bars, and the main channel.