#30 - Rivers like shortcuts too!

Click on the image to enlarge it. No passport needed . . .
This satellite view 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. On the image above the Milk River is sandy green, and meanders from left to right (west to east). The darker-colored segments are oxbow lakes that used to be part of the river.

Not so fast! . . .
Rivers that flow down steeper slopes move faster and tend to cut downward, carving v-shaped valleys. In contrast, streams like the Milk River that flow through fairly flat areas move slower in sweeping bends called "meanders". This is especially true with the Milk River along the Hi-Line where it flows through the wide ancient valley of the Missouri River. Broad flat valleys (floodplains) are formed this way. As 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.

Cut off from the flow . . .
As the river drops sediment at the ends of the abandoned meander it separates that part of the channel from the river, forming an oxbow lake. They are called "oxbows" because they resemble the u-shaped contraptions that farmers put beneath the necks 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. Eventually the lake will disappear as it fills in with organic material and other sediments. You can see some of these oxbow scars on the satellite image atop this page. Watch this animation to see how a slow-moving, meandering river carves a broad valley over thousands of years. IMPORTANT take a few seconds to watch this - CLICK HERE to see a river in Bolivia change over a 28-year period (from 1984-2012).

Term: floodplain

Below: This oxbow lake a few miles west of Harlem is called Dead River. It is a drone photo taken while I was iceskating, looking southwest: Yellow arrows mark the location of the oxbow lake - the skatable former flow of water in the channel that is now Dead River. The short-cut formed at C. It is a 1.7-mile skate from point A to B. Black is the non-skatable portion of the channel (filled with sediment/debris). Blue is the current channel of the Milk River. To access my blog post about a day of skating there (lots of photos), CLICK HERE.