Ellen E. Wohl
Rivers are important components of the water cycle, essential links between the atmospheric and hydrologic processes discussed previously and the massive reservoir of water represented by the world's oceans. North America receives variable quantities of precipitation, mostly from nearby oceans. With the exception of water lost to evapotranspiration and long-term groundwater and glacier storage, North American rivers sooner or later transfer this precipitation back to the oceans. In doing so, rivers perform important additional work in eroding the landscape, including their own channels, and conveying the resultant sediments downstream. Some of this sediment is stored for longer or shorter periods in channel and floodplain deposits, and some sediment reaches the coast and ocean basins fairly quickly. In North America, a small number of major river systems accomplish most of this work, most notably the Mississippi-Missouri-Ohio system, which drains much of the central area to the Gulf of Mexico (table 10.1). Other important rivers, such as the Mackenzie flowing to the Arctic Ocean, and the Colorado draining to the Gulf of California, also contribute significantly to the movement of water and sediment. Even relatively small streams, such as the Eel River in northern California, may transport sediment out of proportion to their basin area. This chapter examines (1) the environmental factors that affect rivers and their channels, and (2) regional differences in river behavior and related issues across the North American continent.
on Channel Characteristics
North America is characterized by a highly diverse array of natural river channels and channel networks. This diversity of channel forms results from diversity in the variables that control channel characteristics. Channel networks respond to the movement of water and sediment from hill slopes and along channels, as controlled by numerous variables. At the broadest level are those variables that influence channel processes over the scale of entire drainage basins and that are largely independent of influences from other variables. These independent variables are (1) the geologic framework of the basin (lithology, structure, tectonic regime), (2) climate, and (3) human land use (fig. 10.1).
The geologic framework determines the substrate on which processes of weathering and erosion act, and consequently the characteristics of water and sediment supplied to the river channel network. North America may be broadly described as having belts of folded and faulted rocks along its eastern and western margins, with a Precambrian Shield and tectonically stable craton in its interior, and recent sedimentary deposits along its southeastern margin (Bally et al., 1989; see chapter 1). Variations in