Basically my brain has taken a leave of absence and won't help me out. I've attached a screenshot of my work so far. I want the mint green polygons to only extend as far as the DEM, which has another transparent polygon drawn over the top of it. How do I go about doing this, essentially the DEM is the maximum extent of my model, (those purple lines and I'm only interested in the city they affect within that zone.
Also as an aside, if anyone knows where I can get concise census data for Tacoma and/or land use breakdowns for the urban area that would be greatly appreciated.