Running Water and Groundwater

Running Water and Groundwater opens with an examination of the hydrologic cycle and the exchange of water between the oceans, atmosphere, and land. The discussion of running water includes an investigation of the factors that control streamflow and their influence on a streams ability to erode and transport materials. Erosional and depositional features of narrow and wide valleys, as well as drainage patterns and stages of valley development, are also described.

After an examination of the importance, occurrence, and movement of groundwater, springs, geysers, wells, and artesian wells are investigated. Following a review of some of the environmental problems associated with groundwater, the chapter ends with a look at the formation and features of caves and karst topography.

Learning Objectives

After reading, studying, and discussing this chapter, you should be able to:

•Describe the movement of water through the hydrologic cycle.

•Describe the process of streamflow and list the factors that influence a stream's ability to erode and

transport materials.

•List and describe the major features produced by stream erosion and deposition.

•Describe the two general types of stream valleys and their major features.

•Distinguish between the different types of drainage patterns.

•Describe how a stream valley and the surrounding landscape can change with time.

•Discuss the occurrence and movement of groundwater.

•Describe springs, geysers, wells, and artesian wells.

•List the major environmental problems associated with groundwater.

•Describe the major features of caves and karst topography.

Chapter Summary

•The hydrologic cycle describes the continuous interchange of water among the oceans, atmosphere, and continents. Powered by energy from the sun, it is a global system in which the atmosphere provides the link between the oceans and continents. The processes involved in the water cycle include precipitation, evaporation, infiltration (the movement of water into rocks or soil through cracks and pore spaces), runoff (water that flows over the land, rather than infiltrating into the ground), and transpiration (the release of water vapor to the atmosphere by plants). Running water is the single most important agent sculpturing Earth’s land surface.

•The factors that determine a streams velocity are gradient (slope of the stream channel), shape, size and roughness of the channel, and the stream’s discharge (amount of water passing a given point per unit of time, frequently measured in cubic feet per second). Most often, the gradient and roughness of a stream decrease downstream, while width, depth, discharge, and velocity increase.

•The two general types of base level (the lowest point to which a stream may erode its channel) are 1) ultimate base level and 2) temporary, or local base level. Any change in base level will cause a stream to adjust and establish a new balance. Lowering base level will cause a stream to erode, while raising base level results in deposition of material in the channel.

•The work of a stream includes erosion (the incorporation of material), transportation (as dissolved load, suspended load, and bed load), and, whenever a stream's velocity decreases, deposition.

•Although many gradations exist, the two general types of stream valleys are 1) narrow V-shaped valleys and 2) wide valleys with flat floors. Because the dominant activity is downcutting toward base level, narrow valleys often contain waterfalls and rapids. When a stream has cut its channel closer to base level, its energy is directed from side to side, and erosion produces a flat valley floor, or floodplain. Streams that flow upon floodplains often move in sweeping bends called meanders. Widespread meandering may result in shorter channel segments, called cutoffs, and/or abandoned bends, called oxbow lakes.

•Common drainage patterns produced by streams include 1) dendritic, 2) radial, 3) rectangular, and 4) trellis.

•As a resource, groundwater represents the largest reservoir of freshwater that is readily available to humans. Geologically, the dissolving action of groundwater produces caves and sinkholes. Groundwater is also an equalizer of stream flow.

•Groundwater is that water which occupies the pore spaces in sediment and rock in a zone beneath the surface called the zone of saturation. The upper limit of this zone is the water table. The zone of aeration is above the water table where the soil, sediment, and rock are not saturated. Groundwater generally moves within the zone of saturation. The quantity of water that can be stored depends on the porosity (the volume of open spaces) of the material. However, the permeability (the ability to transmit a fluid through interconnected pore spaces) of a material is the primary factor controlling the movement of groundwater.

•Springs occur whenever the water table intersects the land surface and a natural flow of groundwater results. Wells, openings bored into the zone of saturation, withdraw groundwater and create roughly conical depressions in the water table known as cones of depression. Artesian wells occur when water rises above the level at which it was initially encountered.

•Some of the current environmental problems involving groundwater include 1) overuse by intense irrigation, 2) land subsidence caused by groundwater withdrawal, and 3) contamination.

•Most caverns form in limestone at or below the water table when acidic groundwater dissolves rock along lines of weakness, such as joints and bedding planes. Karst topography exhibits an irregular terrain punctuated with many depressions, called sinkholes.