Morphology and Flow in Streams

Abstract

Streams are characterized by a continual downstream movement of water, dissolved substances, and suspended particles. These components are derived primarily from the drainage basin or watershed, which is the total land area draining into a given stream channel. Thus the hydrological, chemical, and biological characteristics of a stream reflect the climate, geology, and vegetational cover of the drainage basin [cf., Beaumont (1975), Likens and Bormann (1995), Hynes (1970), Oglesby et al. (1972) and Whitton (1975)]. Water from rain or snow, falling on hilly or mountainous terrain, actually follows diverse routes in moving downhill (Fig. 5.1). Precipitation first may be intercepted by vegetation, then by litter on the surface of the ground. When water is added to the surface of a soil more rapidly than it can soak in (i.e., the infiltration capacity is exceeded) it will run off pverland. Normally, most of the water from precipitation infiltrates into the soil. Soils have variable capacity to store water depending oh depth, structure, composition, and other faqtors. Before stream flow can occur, this storage capacity must be exceeded. Storage capacity jcontinually is made available by evaporation and transpiration (evapotranspiration). Until recently, limnologists have ignored, for the most part, the importance of hydrologic flow paths in regulating the metabolism and biogeochemistry of streams and lakes, as well as their role in the historical generation and accumulation of lake sediments [see Likens (1984)].

Flow through the soil may be channeled by macropores, often produced by cracks, worm or other animal burrows, and old root channels. Impermeable layers can impede the vertical movement of water and cause lateral flow at intermediate depths in the soil (Fig. 5.1). The chemistry of precipitation may be altered considerably as the water passes through the terrestrial ecosystem(s) comprising the drainage basin (Likens and Bormann, 1995; Likens 1984). The surface of the saturated zone of permeable soil is called the water table; water in the soil above the water table is termed vadose water and that below the water table ground water. Ground water provides the relatively stable base flow component in streams. Overland flow, in addition to water that infiltrates the soil and then flows laterally to the stream channel (i.e., subsurface storm flow), are the main components of peak flows or floods [cf., Dunne (1978), Chorley (1978), Winter (1985b)].