Rivers and streams vary widely by lengths, widths, depths, flows, and ecological characteristics. The variability is controlled by the characteristics of the watersheds from which the flows originate (see Figure 1). Rivers and streams range from small, ephemeral, or intermittent streams that flow only in response to rainfall, to rivers such as the Mississippi-Missouri system in the U.S. The Mississippi-Missouri river system has a length of approximately 6690 km (4160 miles) and mean annual flow of 18 400 m3 s−1 (650 000 ft3 s−1 Leopold 1994). Because of the variability, rivers and streams are occasionally difficult to distinguish from other water bodies. Nevertheless, the flowing, running nature clearly separates rivers and streams from other types of water bodies. Rivers and streams are lotic (from lotus, meaning washed) systems that are characterized by running water, as opposed to standing water or lentic (from lenis, meaning calm) water bodies. Rivers and streams generally flow in a particular direction within a definite channel (defined by a thalweg or the line connecting the lowest points in each cross-section) and ultimately discharge into another water body, unless the water evaporates completely. Watershed loading characteristics https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203751510/12ec5900-25b0-4818-a5e2-741994a70951/content/fig3_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> (from USEPA 1992).