Montana Earth Science Picture of the Week

Above: This aerial photo shows the Milk River as it flows past Zurich, a tiny town located 30 miles east of Havre. From its headwaters in Glacier Park, the Milk River flows northeast into Canada, before returning to Montana northwest of Havre. It is a tributary of the Missouri River, and its confluence with the Missouri is located southeast of Glasgow.

Below: The Milk River is shown in yellow. As with most rivers that flow through broad flat floodplains, the Milk River has a tendency to move in sweeping bends called “meanders”. When a river flows around a curve, the fastest water is on the outside of the bend, so erosion is more rapid there. Over time this "sideways" erosion causes the meander of the river to grow wider and wider. Eventually the curve becomes a loop that the river breaks through. Such a shortcut, called a "cutoff", formed at the location marked with the small yellow “X” on the photo below.

As the river drops mud and silt at the ends of the abandoned meander this separates the meander from the river, forming an oxbow lake. They are called "oxbows" because they resemble the oxbows that farmers put over the shoulders of the oxen that pulled plows and wagons. In Australia they are called "billabongs," and in Louisiana they're called "bayous". Since it is no longer part of the river, the waters of the oxbow are calm, allowing a pond or lake ecosystem to develop. In time the lake will disappear as it fills in with organic material and other sediments. Some of the other (unlabeled) oxbow lakes on the photo provide examples of this filling process.

Term: floodplain