This definition is consistent with other definitions developed by interdisciplinary groups of scientists with expertise in riparian issues. For example, Ilhardt et al. (2000) describe riparian areas as “three-dimensional ecotones of interaction that include terrestrial and aquatic ecosystems, that extend down into the groundwater, up above the canopy, outward across the floodplain, up the near slopes that drain to the water, laterally into the terrestrial ecosystem, and along the water course at a variable width.” Lowrance et al. (1985) defines riparian areas as “a complex assemblage of plants and other organisms in an environment adjacent to water. Without definite boundaries, it may include streambanks, floodplain, and wetlands … forming a transitional zone between upland and aquatic habitat. Mainly linear in shape and extent, they are characterized by laterally flowing water that rises and falls at least once within a growing season.” Even programs with disparate goals have developed similar definitions of “riparian.”

Over the last 15 years, several federal agencies have developed considerably narrower definitions of “riparian” for application to their programs, as summarized in Table 1-1. Most definitions reflect the particular goals of individual agencies, including mandates to protect, manage, or restore riparian areas (e.g., BLM and USFS) and to map riparian areas, a responsibility of the U.S. Fish and Wildlife Service (FWS).

It is useful to define two terms sometimes used interchangeably with “riparian.” Almost all rivers have floodplains—aggraded reaches composed of complex bed sediments (alluvia) where flood waters spread out laterally. Clearly part of riparian areas, floodplains are dynamic structures composed of the channel system and adjacent depositional levees, interfluvial bars and low-lying, depositional shelves, often with ridge and swale topography reflecting backfilling of ancient river channels. A river or stream corridor generally refers to riparian areas and their adjacent waterbodies as a unit defined longitudinally from headwaters to the ocean. Figure 1-4 is a schematic of a stream corridor that shows the many interconnections between its different components. Because floodplains are porous and contain aquifers that are closely linked to and controlled by the channel system, waterbodies and their riparian areas are linked longitudinally, vertically, and horizontally—not just by the movement of water and sediments, but also by the movement of biota (Stanford and Ward, 1993).