Deforestation of riparian zones is known to influence the numbers and kinds of oranisms that inhabit adjoining streams, but little quantitative information is available on how much deforestation must occur before the biota is affected. [

The hydrologic regime of the Illinois River has been altered over the past 100 years. Locks and dams regulate water surface elevations and flow, enabling commercial navigation to continue year round.

Hydrologic alteration is indisputably the most cited cause of species decline in aquatic systems. Most studies focus on the longitudinal impacts of dam construction and operation, but few examine the impact of flow alteration on tributaries. Consequently, tributary habitat and fauna are often overlooked in management processes involving mainstem projects. Most tributary impacts fall into two categories described by the National Environmental Policy Act (NEPA): direct effects and cumulative impacts. This review summarizes the issues that are most pervasive in the literature, provides specific examples of known effects, and integrates complex ecological aspects relecant to the hydropower relicensing process.