GROUNDWATER AND RIVER FLOW

GROUND WATER AND RIVER FLOW [This article, written by the American Ground Water Trust was originally published in

THE AMERICAN WELL OWNER, 2003, Number 3]

GROUND WATER AND RIVER FLOW

Where does the water in a stream come from during a drought or when it has not rained recently? Base flow is the technical name for the dry weather flow in a stream or river. River base flow results from ground water seeping into riverbanks or the riverbed. The flow may be significant enough to allow the stream to flow year round (i.e., perennial or permanent stream). Without base flow recharge from ground water to streams and rivers, many would not carry a flow of water except during storms. Streams that flow only periodically in response to rainstorms or seasonal snowmelt events are known as ephemeral or intermittent streams. On average, 40 percent of all flow in United States rivers and streams originates as ground water. Trout streams that flow year round with cool clear water virtually all result from constant input from ground water.

Water flowing into a stream from ground water is called a “gaining stream” and this is the most common occurrence. However, there are also “losing streams” that “leak water from their channel into the ground beneath. Losing streams are common in dry environments where ground water may flow in a stream only during the “rainy season” of the year. In a gaining stream, the ground water level is higher than the water level in the channel. In a losing stream, the ground water is below stream level.

When river levels rise, for example in response to a storm, water can flow from the river into the channel banks as the water level in the channel rises above the pre-storm ground water level. If the stream over tops its banks to spread over a flood plain, flood water infiltrates to the ground water under the flood plain. This seepage and infiltration can help reduce the impacts of flooding in downstream areas, and after the storm, the slow release of water from the surrounding saturated area maintains the base flow in the channel. Infiltration through the flood plain to the underlying ground water table is one of the reasons why maintaining flood plains in an undeveloped (pervious) condition must be an important consideration for planning development. Tidal rivers may also induce a pattern of losing and gaining conditions as the elevation of the water in the channel rises and falls twice per day with the tide.

A stream may switch back and forth between losing or gaining on a seasonal basis during the year and/or during the course of its flow downstream from its headwaters. Conditions may change from gaining to losing at the upstream end of a meander or at the top of an abrupt change in the gradient of the channel. Pumping a well in the vicinity of a stream or lake may induce a “losing” condition when the zone of drawdown around the well intersects the surface water body.Ground water and surface water are not separate resources. When our activities use one of these resources, it often affects the other in a relatively short time frame in terms of quantity and quality.

[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2003, Number 3]