INTRODUCTION

Many of the challenges and opportunities articulated in international water assessments of the 1990s indicated a growing recognition of ecosystem changes generated by human water use. Much knowledge has been gained in recent years about the human effects on aquatic, riparian, estuarine, and groundwater ecosystems. Some effects, such as eradication of native non-commercial fish, purposely accelerated environmental impacts, while others have occurred with little or no recognition until illuminated by scientific or public inquiry (Gosnell, 2001). Assessment of habitat conditions, along with distinctions among levels of species endangerment, has become increasingly refined (IUCN, 2000). But many human effects on aquatic, amphibian, invertebrate, and fish species and their habitats still lie beyond comprehension.

Each year new surprises occur: an amphibious population declines, a waterbird species experiences birth defects, an exotic mollusk invades new waters, a fish disease spreads, an insect disease vector adapts to control methods. With sufficient time, the surprise is recognized, management alternatives are formulated, and actions are taken. For example, the adverse water quality effects of irrigation return flows from pesticide contamination in the Aral Sea basin of Central Asia, like increased selenium concentrations from irrigation return flows to the Kesterson Reservoir of California, have opened up new lines of environmental impact assessment and adjustments in irrigated areas of the world (Ghassemi et al., 1995; Kobori and Glantz, 1998; Micklin and Williams, 1996; NRC, 1989b). Remote sensing and aerial photography provide stark images of those impacts (Figure 3.1).