Many Perennial streams in Virginia's Ridge & Valley geological areas begin in mountain watersheds in Forested areas and then flow out into valley areas with Karst geology where they may go underground for part of the year.
Therefore it is important that the mountain streams in forested areas are shown in enough detail on at least one of the data sets that DCR uses, so that a stream that goes underground into the Karst geology of the Valley areas, can be considered to be a Perennial stream based on its upstream portion that has riffles and pools.