The Mighty Mo!

One of our blogs was to be based on a popular science magazine, but because I would like to focus on the South Dakota Missouri River flooding in 2011, I was unsuccessful in finding such an article. I did however find an article written by Tim Cowman of the Missouri River Institute at the University of South Dakota. This article was excellent in describing many effects of the flooding and answered several of my questions. While this article detailed mainly the section of the river between Fort Randall and Gavins Point Dams, much of it can be applied elsewhere.

First, I wanted to understand how the channel was changed during the floods, as sandbars grew and changed shape or location. The river bed is generally unconsolidated silt, sand, and clay, which is easily moved at slower (or regular) velocities. So the high velocities during the flood eroded, carried, and deposited much larger volumes of sediment. According to the article typically dunes will build up to within two or three feet of the river level, and the levels rose to about 10 feet above normal levels. As the levels dropped and velocities slowed, the larger sediment load was deposited leaving larger dunes at several feet above normal river levels. So where did the larger sediment load come from? It seems the river channel is reportedly deeper then before in some areas leading to the conclusion of riverbed erosion. Also bank erosion was observed but to a lesser extent. The channel location moved in several reported areas, changing the flow rates, channel patterns, and surrounding landscape.

I was also curious about how flooding effected the surrounding ecology, which was addressed in the article as well. It was stated that changing flow rates can effect fisheries and wildlife patterns, changing established habitats and possible river crossing points for wildlife. The deeper channels drain surrounding wetlands that provide habitat for many species and helps control future flooding. Deposition has covered some wetlands and cropland hindering their original purpose. Cottonwood trees are an important factor in riparian forests and can withstand short periods of water inundation but longer periods observed during the flood can damage root systems. Undercutting of banks by floodwaters also knock over cottonwoods. On the positive side, downed cottonwoods can create an ideal habitat for certain fish and insects species, and the new sandbars and floodplain deposits can provided area for new cottonwood growth. Undesirable species such as Eastern red cedar and Russian olive trees cannot tolerate standing water, thus removing them from the area. Invasive species can fortunately also be wiped out of riparian areas, such as the mentioned purple loosestrife. This species will take over an area, not allowing other native species to grow. Unfortunately its seed can be carried by floodwaters and deposited to create new colonies.

Since a flood of this magnitude has not been recorded since the dams were built in the 1950s, many of the long term effects are unknown. Many of the above mentioned impacts will need to be monitored and it may be many years before the total effects will be known.

The article in its entirety can be found at the following link.

http://www.usd.edu/missouri-river-institute/upload/Impactsof2011Flood.pdf