Doug, from what I understand, you are using a DEM surface raster, clipped to the extent of a lake (6000 x 3500 m) as 'input pour point raster or feature data' to the 'Snap pour point' tool? I am not sure if this would work for what you are trying to create.
If your objective is to create a watershed for a Lake, I would go about the process in this way. I'll demonstrate the process using an example lake in Stowe, Vermont.
- This is the example lake for which watershed is desired
- Start with input DEM surface raster.
1) Run Fill tool to remove any sinks or imperfections. Output is Filled Surface Raster.
2) Run Flow Direction tool on the Filled Surface Raster to create D8 Flow Direction Raster.
3) Run Flow Accumulation tool using the D8 Flow Direction Raster as input to create D8 Flow Accumulation Raster. The cells of high flow accumulation (this is where streams form) should go through your lake. Adding some transparency to the output Flow Accumulation Raster, we can see that cells of high flow accumulation (seen in white) flow through the lake.
4) Create a new point feature class near the downstream end of your lake, which we will provide as input to the snap pour point tool. Using a point further downstream would create a watershed for the entire lake, including all regions that drain into the lake.
5) Use the snap pour point tool. Input pour point feature data is the point you just created. Input accumulation raster is the Flow Accumulation Raster created from the flow accumulation tool. Provide a conservative search distance by approximately measuring the distance between the point you created to a cell of high flow accumulation. In this case I will use 200 meters.
6) Run the Watershed tool. Input D8 flow direction raster is the D8 Flow Direction Raster and Input pour point data is the output snapped pour point created by the snap pour point tool. This creates a watershed for the entire lake. Apply some transparency to the output Watershed to see the regions encompassed by the watershed, which include the entire lake.
- Hope this addresses your question.
Thanks,
Neeraj