INTRODUCTION: Bridges serve a variety of highway purposes including the elimination of conflicts with traffic and other modes of transportation@ such as rail@ marine@ air@ and pedestrian. Bridges enable watercourses to maintain the natural function of flow conveyance and sustain aquatic life. Bridges are also important and expensive highway-hydraulic structures and are vulnerable to failure from flood-related causes. To minimize the risk of failure@ the hydraulic requirements of stream crossings must be recognized and considered in all phases of highway development@ construction@ and maintenance. Features that are important to the hydraulic performance of a bridge include the approach fill alignment@ skew@ and profile; bridge location@ skew@ and length; span lengths; bent and pier location and design; and foundation and superstructure configuration and elevations. These features of a highway-stream crossing are usually the responsibility of location@ design@ and bridge engineers; however@ the integrity and safety of the facility are often as dependent upon competent hydraulic design as on competent structural and geometric design. In this chapter@ the hydraulic engineering aspects of bridged stream crossings are discussed@ proceeding from hydraulic considerations in planning and location through studies necessary for design and construction to hydraulic considerations in maintenance and operation. Tidal areas@ such as bays and estuaries@ are not discussed in detail although much of the discussion is applicable to these areas. Another chapter will discuss tidal hydraulics. Structures measuring more than 6.1 m (20 ft) along the roadway centerline are conventionally classified as bridges. However@ structures designed hydraulically as bridges are treated in this chapter@ regardless of length. Discussion of the hydraulics of bridges will include consideration of the total crossing@ including approach embankments and structures on the floodplains.