• 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 stream's 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 downcut, 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.

  

• Floods are triggered by heavy rains and/or snowmelt. Sometimes human interference can worsen or even cause floods. Flood-control measures include the building of artificial levees and dams, as well as channelization, which could involve creating artificial cutoffs. Many scientists and engineers advocate a nonstructural approach to flood control that involves more appropriate land use.

• The land area that contributes water to a stream is called a drainage basin. Drainage basin are separated by an imaginary line called a divide. 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 that 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.

• The quantity of water that can be stored depends on the porosity (the volume of open spaces) of the material. The permeability (the ability to transmit a fluid through interconnected pore spaces) of a material is a very important factor controlling the movement of groundwater.

• Materials with very small pore spaces (such as clay) hinder or prevent groundwater movement and are called aquitards. Aquifers consist of materials with larger pore spaces (such as sand) that are permeable and transmit groundwater freely.

• 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.

• When groundwater circulates at great depths, it becomes heated. If it rises, the water may emerge as a hot spring. Geysers occur when groundwater is heated in underground chambers, expands, and some water quickly changes to steam, causing the geyser to erupt. The source of heat for most hot springs and geysers is hot igneous rock.

  

• 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.

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