Use flow accumulation grid rather than STR900 grid

[USGSPMM3]

Add the flow accumulation grids (state based) to the map services and then write a filter/slider bar that shows only cells having accumulation greater than x, where x is determined by the user. For example, the STR900 grid is the accumulation of 900 cells or more which is the flow accumulation grid of cells >900. By using the flow accumulation grid the user is given the capability of individualizing their STR grid to be STR100 or STR5000.

We may want to limit the lower end so they can't go below a certain threshold. I think each state should also have a "default" threshold which will depend on the state resolution of data. For example, a 10-meter grid would use 900 cells, a 3-meter grid might use 9,000 cells, etc.