PUB00070 - Laserfiche WebLink

Home Browse Search

Streams flowing from mountainous terrain commonly flow across alluvial fans at the edges of the valleys, Most streams in this type of setting lose water to ground water as they traverse the highly permeable alluvial fans, This process has long been recognized in arid western regions, but it also has been documented in humid regions, such as the Appalachian Mountains, In arid and semiarid regions, seepage of water from the stream can be the principal source of aquifer recharge, Despite its importance, ground-water Mountain stream in Oregon. (Photograph by Dennis Wentz.) recharge from losing streams remains a highly uncertain part of the water balance of aquifers in these regions, Promising new methods of estimating ground-water recharge, at least locally, along mountain fronts are being developed-these methods include use of environmental tracers, measuring vertical temperature profiles in stream- beds, measuring hydraulic characteristics of streambeds, and measuring the difference in hydraulic head between the stream and the underlying aquifer, The most common natural lakes in moun- tainous terrain are those that are dammed by rock sills or glacial deposits high in the mountains, Alluvial fan in Alaska. (Photograph by Earl Brabb.) Termed cirque lakes, they receive much of their water from snowmelt. However, they interact with ground water much like the processes shown in Figure 21, and they can be maintained by ground water throughout the snow-free season, The geochemical environment of mountains is quite diverse because of the effects of highly variable climate and many different rock and soil types on the evolution of water chemistry, Geologic materials can include crystalline, volcanic, and sedimentary rocks and glacial deposits, Sediments can vary from those having well-developed soil horizons to stream alluvium that has no soil development. During heavy precipitation, much water flows through shallow flow paths, where it interacts with microbes and soil gases, In the deeper flow through fractured bedrock, longer term geochemical interactions of ground water with minerals determine the chem- istry of water that eventually discharges to streams, Base flow of streams in mountainous terrain is derived by drainage from saturated alluvium in valley bottoms and from drainage of bedrock fractures, Mixing of these chemically different water types results in geochemical reac- tions that affect the chemistry of water in streams, During downstream transport in the channel, stream water mixes with ground water in the hyporheic zone, In some mountain streams, the volume of water in the hyporheic zone is consider- ably larger than that in the stream channel. Chem- ical reactions in hyporheic zones can, in some cases, substantially alter the water chemistry of streams (Figure 19), 35