Geologic hazards of the Bighorn River Floodplain: Meandering Streams

In a previous article I described how the Wind River is an antecedent stream which pre-existed the Owl Creek Mountains. As the mountains were uplifted during the Laramide Orogeny, the river maintained its grade by down-cutting and creating the Wind River Canyon. In addition to maintaining the river’s grade, the river’s course was also maintained. The sinuous curvatures of this river’s course classifies the river as a “meandering stream.” The curves we enjoy when driving through the Canyon are the same meander curves that the river had over 70 million years ago. How cool is that!

The Bighorn River, free of the confines of the Wind River Canyon at the “wedding of the waters,” is free to meander without restriction. When streams are mature and in equilibrium with their grade, they naturally meander back and forth and create a floodplain. There are two basic mechanisms by which this meandering takes place. The most common is that the moving water tends to erode the bank upon which it impinges in a meander curve and deposits sediment on the opposite bank. Over time this causes the stream course to migrate in the direction of erosion until it reaches the edge of the floodplain. At this point, the stream may erode the edges of the floodplain and expand it. Ultimately though, it will start to migrate downstream and in the other direction via the same process of erosion and deposition. This mechanism of meandering is relatively slow and predictable. Where the Bighorn River flows past the Coachman Motel and south of Shoshone Street is an area of erosion such that the river’s course is slowly moving in a northerly direction towards town.

The second mechanism of a meandering stream is more dramatic and sudden. During periods of flooding, the entire floodplain can be inundated by rapidly flowing water. Instead of the erosion and deposition mechanism of meandering, the stream can take the shortest route available establishing a new course segment. The new course segment completely bypasses a meander curve. This change of river course is both sudden and potentially devastating. The topographic map of the Thermopolis area shows a possible shortcut for the Bighorn River through the area occupied by the water plant where the meander curves are closely spaced.

Where the river passes through town, it is a fairly straight, north-flowing segment without meander curves. There is a notable floodplain restriction in the area of the swinging bridge. This restriction is of concern and will be discussed in a future article.