Areas draining into cells with a value of nodata were not filled by this procedure. After the FILL procedure, the original elevations were replaced into the cells that had been set to nodata. The two filled DEM's were trimmed to have a 12-kilometer overlap. To save disk space, the DEM elevations were rounded to the nearest meter after calculation of flow directions. A statewide grid of accumulated flow was generated from the flow-direction grid.
Accumulated flow is the number of cells flowing into each cell in the output grid.
Cells with high accumulated flow values may be used to identify stream channels. Cells with accumulated flow values of zero are local topographic highs and can be used to identify ridges and drainage basin boundaries. To save disk space, the accumulated flow data were reclassified to five categories. The flow-accumulation data set was used in the selection of watershed pour points or outlets. Outlets were selected at stream confluences in cells with high accumulated flow.