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Lake Learning column: Where does my lake water come from?

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All water on the globe whether in the atmosphere (clouds), on the ground (rivers, lakes, ocean), or under the ground (ground water) is ultimately connected. These different groups of connected water can be broken down into smaller and smaller divisions that are more intimately connected.

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Today I will explain watersheds, which are a regional division of water connection. The size and location of watersheds have a lot to do with lake types. Before I get ahead of myself, let's step back and define watersheds. A watershed is an area of land where the water all drains into the same river system. Creeks flow into streams, which flow through lakes, which ultimately end up in major rivers that drain into the ocean. You can picture it like branches of a tree going down into the trunk. These watershed areas are defined by topography, or ridges of elevation. Therefore, watersheds are mainly driven by gravity -- water runs down hill.

Minnesota is split into 8 major water basins, the two in this area are the Red River Basin and the Upper Mississippi Basin. These major basins are composed of major watersheds. In Becker and Ottertail Counties these major watersheds are the Buffalo River and the Ottertail River. And of course, the major watersheds are composed of numerous minor watersheds, including the Pelican River Watershed.

Each lake also has an immediate watershed, which includes stream inlets and runoff from adjacent land. The size of an immediate lake watershed and the land use in that watershed affect the lake's condition. Also, something that happens upstream in a watershed can affect everything downstream from it like a domino effect.

Usually, as the ratio of watershed area to lake area increases, the water quality declines. If a lake has a very small watershed or is at the top of a watershed (in topography terms), the lake usually has better water clarity than a lake at the bottom of a large watershed. As water flows downhill through a watershed it picks up sediment from erosion and nutrients from runoff. This sediment and nutrients can feed algae and cause the lake to become "greener".

Land use in a watershed affects lake water quality as well. To compare, an area of 24 acres made up of forest contributes approximately 4.6 kg of phosphorus a year, while an area of 24 acres made up of rural/agriculture contributes about 30 kg of phosphorus a year and the corresponding urban contribution from the same area is 50 kg of phosphorus a year. Therefore, cutting down forest to create an urban area translates into a considerable change in phosphorus runoff to adjacent lakes. The reason urban area runoff is so high is because of the large amount of impervious surface, which is surface that does not allow water to soak into the ground such as roads and buildings.

To find the types of land use in your watershed on a map, for Becker County visit: http://gis-server.co.becker.mn.us/website/beckerpublic/disclaimer.htm and for Ottertail County visit: http://www.co.otter-tail.mn.us/maps/.

The MN DNR is in the process of the Minnesota Lake Watershed Delineation (Lakeshed) Project, which is defining watersheds for all lakes in the state that have a surface area of 100 acres or more. To learn about this project and look at the maps, visit: http://www.dnr.state.mn.us/watersheds/lakeshed_project.html.

Until next week, enjoy the lakes!

(Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com.)

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