Here the big meanders of the Red River of the North through Crookston are evident. The water moves slower in a sinuous channel than it does in a straightened channel because it has to travel farther.

Karen Terry, University of Minnesota Extension Service, 888-241-0843

In the last issue, rivers were described as ecosystems that can be artificially broken down into five categories to make them easier to study. Those categories are hydrology, biology, water quality, geomorphology, and connectivity. The first three were discussed in the last issue. This article will focus on geomorphology, and connectivity of rivers will be covered in the next issue. As a reminder: there is a lot of overlap and interdependence among these five categories so any change to the river system typically results in impacts that fall into more than one category.

Geomorphology refers to the shape of the river. This is determined by several factors such as the amount of water flowing through it, the steepness of the river valley, and soil type. Think about the streams on Minnesota’s North Shore: they tend to be fairly straight, with few meanders, because the slope is steep and they are dominated by boulders and bedrock. They are more like mountain streams than prairie streams. Prairie streams, like those in southern and western Minnesota, tend to be low gradient (not steep), and the soils are loamy and fine. These factors create streams that are curvy, or that meander back and forth across the landscape, and often have broad floodplains. In addition to meandering across the floodplain, rivers also move up and down along their beds, forming deep places (pools) and shallow places (riffles).

This aerial photo shows a sinuous stream in northwestern Minnesota. Stable streams have predictable patterns, including size and spacing of meanders.

Although healthy rivers have predictable shapes, they do change over time. River bends, and the pools and riffles associated with them, tend to move downstream, and oxbows are sometimes created when a river abandons a bend by cutting a new channel. These changes, when not impacted by humans, happen very slowly and the river’s overall geomorphology remains the same.

Many of Minnesota’s streams have been altered by humans. Practices such as channelizing, which is removing the meanders and creating a straight ditch, and dredging, which is removing the shallow areas to make the river uniformly deep, disrupt the river’s ecosystem. These altered systems have less diverse habitat, which negatively affects the health of the plant and animal communities that live there, and the altered systems are unstable because the river’s physical properties continuously work to return to the stable geomorphology that existed prior to the disturbance. This is often visible as raw, eroding banks as the river eats away at the straightened edges to re-establish its meanders. This exacerbated erosion contributes huge amounts of sediment to the river. Channelization, because it steepens the slope of the river, also impacts the hydrology of the system by speeding up the water.

In the next issue, connectivity, the last of the five components of healthy rivers will be discussed.

Posted in Lakes |