Pomme de Terre Watershed District
What did the fish that was swimming upstream say when he ran into something?
I know, it's a lame joke, but it relates to the theme of this article, river and stream connectivity. The term connectivity refers to the flow, exchange and pathways that move organisms, energy and matter through a stream system. This article will focus on two types of connectivity in a river. Longitudinal connectivity is the connections of a river from its headwaters all the way down to its mouth. Lateral connectivity is the connection between the stream channel and its floodplain. Both are crucial to stream health and stability.
Fragmenting a stream with dams and culverts disrupts the longitudinal connectivity of a river. Uninterrupted flow along the entire length of a river is essential for the proper flow and exchange of water, energy, sediments, nutrients and organisms. Structures that fragment streams disrupt the progression of stream habitats from small, shaded, headwaters streams to large, flat slow flowing valley streams. In Minnesota more than 900 dams greater than six feet high and hundreds of smaller low-head dams have been built on state rivers and streams.
The ecological consequences of fragmenting a stream with physical barriers include: blocking fish movement and migration; blocking the downstream movement of sediment; disrupting nutrient and energy cycles; and modifying thermal and flow regimes, often to the detriment of existing species.
Appleton was the site of a mill dam on the Pomme de Terre which was originally built in 1872. By 1997, the reservoir behind the dam was filled with over 12 feet of sediment. (The flood of 1997 caused the dam to partially fail.) Due to the cost of repairs to modern standards, the loss of reservoir functions due to the sediment build up, and recognition of ecological damages caused by the dam, the City of Appleton and the Minnesota DNR decided to remove the structure. DNR fisheries crews compared fish surveys done prior to the dam removal with surveys done in 2001 after the dam was removed. With the dam in place, the fish community above the dam was dominated by black bullheads. After the dam was removed the fish community dominance shifted to more riverine species like drum, channel catfish and walleye.
In 1939, the Works Progress Administration built the Marsh Lake dam near the mouth of the Pomme de Terre River. As part of this project, the Pomme de Terre was actually rerouted from below the dam into the reservoir above the dam. Creation of the reservoir and rerouting of the Pomme de Terre River increased reservoir fish and wildlife habitat and created new colonial water bird habitat. It also disrupted natural river functions and processes, affecting sediment movement and floodplain function, blocking fish movement, and reducing riverine and floodplain habitats. Natural flooding and drying cycles were disrupted, reducing emergent aquatic plants and associated fish and wildlife habitats found in the area prior to impoundment. Currently the US Army Corps of Engineers and the Minnesota DNR are conducting a feasibility study on what should be done with the Marsh Lake dam, which includes possibly putting the Pomme de Terre back into its natural channel on the downstream side of the dam which would restore river habitat and provide for game fish migration.
As I mentioned in a previous article, the stream channel needs to be connected with its floodplain. This is lateral connectivity. Flood plains play an important role because this land reduces the floodwaters energy with plants and trees. The great flows are allowed to spread out and slow down. The floodplain provides temporary storage for floodwaters and sediment is allowed to settle out. Floodwaters nourish floodplains with sediments and nutrients and provide temporary aquatic habitats for invertebrate communities, amphibians, reptiles and spawning fish.
Floodplains converted to urban areas or farmland do not effectively dissipate or store floodwaters. Riparian zones that are farmed, mowed, grazed, deforested, or developed replace natural vegetation with crops, lawns, bare soil and pavement. These areas are called floodplains for a reason... they flood. Millions and millions of taxpayer dollars are spent to keep these areas from flooding. It will be a never ending fight. Levees and dikes will be built and raised. These further disconnect the river from its floodplain. The channel is forced to carry the flood flows, leading to greater in-channel erosion and damage. Constraining the channel leads to higher river stages leading to greater flooding downstream.
About the author: Shaun McNally is the Pomme de Terre Watershed Project Coordinator. He is located in the Stevens SWCD office in Morris and can be reached at 320-589-4886 ext. 109.
The Pomme de Terre River Association maintains a website: www.pdtriver.org.