CLOGGING THE MISSISSIPPI? GLACIAL BEDLOAD FROM THE ROCKIES TO THE GULF

Although most of the work done by rivers is generally concluded to be the transport of suspended load, there is little question but that the load component that best explains the evolution of fluvial systems is bedload. Bedload is generated by the weathering and erosion of headwaters basins, and is transported to upper alluvial reaches during events such as glaciation. The reworking of pulses of bedload, and the effects of such pulses on fluvial systems, has been studied since Gilbert (1917). However, there is no consensus as to the spatial and temporal integration of such pulses into the larger basins they feed.

I use an empirical “sandbox” approach to analyze the spatial and temporal integration of sediment pulses into major headwaters tributaries of the Mississippi-Missouri system; specifically the Madison (Turner, 1995), Shoshone and Yellowstone Rivers. In 40 years the 1959 Madison ‘Slide has been incised, sediment deposited below the slide and itself progressively incised. The locus of deposition has migrated about 3 km downstream from the slide and is converging with the pre-slide river profile. It is unlikely that any morphological evidence of the slide upon the river will ever be formed beyond 5 km downstream, and it will likely require tens of thousands of years to reach that point.

The Shoshone River proceeds without significant tributaries from 25 km below its lastglacial terminal moraines to 120 km downstream, where it joins the Bighorn/Wind River/Yellowstone system. The lastglacial outwash terrace, over 100 m above the Shoshone at the moraines, converges with it ~90 km downstream. The locus of deposition has migrated that far in ~15,000 years. Similarly, the upper Yellowstone receives load from tributary after tributary draining its mountainous headwaters. Although the contribution of each tributary cannot be tracked, the low lastglacial terrace converges with the river downstream from the last tributary.

In summary: abundant bedload forces major changes in headwaters river channels, dominantly in morphology and gradient. However, any point supply of load is finite and is fully assimilated near the mountains. Downstream systems such as the mainstem Mississippi River experience those headwaters signals only as a minor long-delayed rise in bedload, if at all.