Determining the extent and condition of the nation’s riparian areas is fundamental to managing them for multiple purposes. Although snapshots in time of riparian areas are useful for knowing their present status, the true utility of acreage and condition information lies in observing trends in these data over time and in understanding the factors causing such trends. Trends information is critical for relating riparian conditions (e.g., wildlife populations and vegetation) to other factors such as human population growth and water use. Such information can be used to predict future conditions in the presence or absence of restoration activities. And it can motivate decision-makers to take action before riparian areas are irreversibly impacted or destroyed. Surprisingly, there have been very few assessments of riparian acreage across the United States and only a handful of comprehensive studies on the condition of riparian lands.
The amount of total land classified as “riparian” obviously depends on one’s definition of that term. Indeed, variable definitions partially account for the inconsistent data found in reports of riparian acreage across the country. Many reports measure riparian areas in stream miles rather than acres, making direct comparisons difficult. Figure 3-14 shows the distribution of estimated stream miles and riparian acreage across the United States. National Resources Inventory and U.S. Environmental Protection Agency (EPA) estimates of current riparian acreage—which assume that the riparian area extends 50 ft from the edge of waterbodies—are 62 million and 38 million acres, respectively (excluding Alaska). Brinson et al. (1981) estimates a liberal upper limit of 121 million riparian acres, which includes all land in the 48 contiguous states that is within the 100-year floodplain and is thus potentially able to support riparian vegetation. This estimate was refined by Swift (1984) to those areas within the 100-year floodplains of streams and rivers that have certain vegetative characteristics. Swift estimated that there were at least 67 million acres of riparian land in the United States prior to European settlement, with about 23 million acres remaining. Reasons for the decrease in acreage include removal of vegetation along streambanks, channel straightening to remove meanders, and flooding of riparian areas upstream of impoundments. For example, an estimated 70 percent of the original floodplain forests have been converted to agricultural and urban land uses. Brinson et al. (1981) estimated that impoundments alone had inundated more than 24,000 km of streams, while the downstream effects of modified streamflow on riparian functions have been seldom documented. Case histories show that in some areas loss of natural riparian vegetation is as much as 95 percent—indicating that riparian areas are some of the most severely altered landscapes in the United States (Brinson et al., 1981).