Headwater Stream Ecosystems: An Initial Evaluation of Their Threat Status

Abstract

Headwater streams are the first expression of a flowing-water ecosystem (between a spring and a small stream), and in many regions likely have intermittent or ephemeral flows. These ecosystems have very high edge to area ratios, and therefore are extremely sensitive to land-use effects, at reach and catchment (watershed) scales. The downstream limits of headwaters have been defined in different ways, but they are nevertheless the smallest streams in any fluvial network globally. Their small size also means they are relatively easy to drain, dam, redirect, or ignore in terms of protective measures. Headwaters can occur in any landscape, including mountains, plains, and lowlands. They support a great diversity of life that is found nowhere else downstream in their stream networks, and many of their species are themselves in peril. The lack of agreement on the extent to which headwaters are separate from other small streams, and the downstream boundary of headwaters means that their total length, and portion of that length that is threatened, is difficult to tally in any landscape but it is seldom insubstantial. However, since smaller headwater streams dominate (by length) the world's running waters, and provide important sources of water, sediments, and biota, they deserve comprehensive protection.