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I am trying to delineate just the catchment area of some lakes, and am wondering the correct way to do this. I already have a filled DEM, along with a flow direction and a flow accumulation raster. Is there a way to accomplish this task without the use of pour points? Or do I have to use them. If that is the case, can a pour point be located in the middle of a lake (assuming that the middle of a lake is the highest area of flow accumulation). Whenever I do try to run this data, whether with or without a pour point, I get something that does not look completely right. The catchment area does not encompass the entire lake, and the actual watershed itself does not accurately represent the flow accumulation arrows. Am I doing something wrong, or is there a simple cause to my problem. Maybe if someone could take a quick second to step through the watershed toolset, that would be helpful. Am using ArcGIS 10 SP1, with Spatial Analyst, ArcView license.

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Does water flow out of the lakes? If so have you located the pour points on the outflow stream? –  Dan Patterson Feb 15 '11 at 23:04
    
Hey Dan. I tried what you suggested with the pour point at the outflow stream, and it worked. The only thing is, I only want what drains to this particular lake, so I assuming that I can 'burn' an artificial dam into the inflow areas surrounding the lake, so I can get just the particular lake, and not everything that flows through that lake from higher elevations. Correct me if I am wrong. –  Andrew Feb 16 '11 at 14:45

1 Answer 1

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The way that ArcGIS computes watersheds probably doesn't match with your intuition: only areas upland of a specific pourpoint will drain to that spot, so if you locate the pour point at the center of the lake, 'lower' elevation spots drain into other basins. As Dan mentioned, instead you want to locate it at the top of the outflow stream, where the water from the lake heads to rivers below.

Another option to produce this effect is to continue locating the pour point in the center of the lake, but build an artificial 'dam' at the bottom of the watershed by 'burning': create a new line feature, convert it to a raster with a large value (say, 300). Now the DEM should be forced to empty at your desired pour, with the only artifacts being introduced by the manually produced dam itself.

If you're looking for something more sophisticated, you may want to look into the ArcHydro tools (beta version for ArcGIS 10), which provide a more detailed set of tools for working with hydrologic features -- see this tutorial for details.

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I will give that a try. Thanks for the insight. I've never tried 'burning' before, but it seems to make sense. As I commented above, I will try your method to stop the flow of water into the lake from surrounding sub-watersheds, just to get the drainage area for the lake. –  Andrew Feb 16 '11 at 14:46
    
The 'burn' worked. Thanks for your help!!! –  Andrew Feb 16 '11 at 17:38

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