The Federal Highway Administration document "Highways in the River Environment - Hydraulic and Environmental Design Considerations" was first published in 1975, was revised in 1990, and is now issued as Hydraulic Design Series 6, "River Engineering for Highway Encroachments." This document has proven to be a singularly authoritative document for the design of highway associated hydraulic structures in moveable boundary waterways. This revised document incorporates many technical advances that have been made in this discipline since 1990. In addition, Hydraulic Engineering Circulars (HEC) 18, 20, and 23, have been published since 1990. This document and the HECs provide detailed guidance on stream instability, scour, and appropriate countermeasures. In HDS-6, hydraulic problems at stream crossings are described in detail and the hydraulic principles of rigid and moveable boundary channels are discussed. In the United States, the average annual damage related to hydraulic problems at highway facilities on the Federal-aid system is $40 million. Damages by streams can be reduced significantly by considering channel stability. The types of river changes to be carefully considered relate to: (1) lateral bank erosion; (2) degradation and aggradation of the streambed that continues over a period of years, and (3) natural short-term fluctuations of streambed elevation that are usually associated with the passage of floods. The major topics are: sediment transport, natural and human induced causes of waterway response, stream stabilization (bed and banks), hydraulic modeling and computer applications, and countermeasures. Case histories of typical human and natural impacts on waterways are analyzed.