Groundwater seepage into streams keeps water in them during droughts.

Atlanta, Georgia was in the midst of a severe drought in 2008, and, as the left side chart below shows, virtually no rainfall fell during September. At the Peachtree Creek site near downtown, only 1/10th of an inch of rain fell, on about Sept. 8th. Certainly the water flowing in Peachtree Creek comes from rainfall, but if it depended on only rainfall runoff for its water supply, the creek would have been dry as a bone the whole month. Next, look at the right-side chart below; it shows the gage height (river stage) of Peachtree Creek for the same time period. Gage height is a somewhat arbitrary datum height, in feet, above an altitude designated as zero. The zero point is set to be below the base-flow height of the river. For Peachtree Creek, the level of the water during periods of no rainfall (its base flow) is about 2 feet.