The stream simulation approach for designing road-stream crossing structures developed and used by the U.S. Forest Service provide a pragmatic and sustainable long-term solution to maintain passage for all aquatic organisms at all life stages, while meeting vehicle transportation objectives. The premise of stream simulation is that by creating similar channel dimensions and characteristics in the structure as those in the adjacent natural channel, fish and other aquatic organisms should experience no greater difficulty moving through the structure than if there were no crossing. The stream simulation design approach is an interdisciplinary process, integrating concepts of fluvial geomorphology with engineering principles to design a natural and dynamic channel through a rigid structure. Using channel characteristics from an adjacent representative reach as a starting point to develop the stream simulation design channel width, bed material sediment sizes, and bedform types and spacing are verified and adjusted to provide continuity of flow hydraulics, sediment transport, and ultimately aquatic organism movement through the crossing for a wide range of flows. Because the dimensions and characteristics of stream simulation designs are similar to those in the adjacent natural channel, stream simulation channels are capable of laterally and vertically adjusting to a wide range of floods and sediment/wood inputs without compromising the movement needs of fish and other aquatic organisms or the hydraulic capacity of the structure. Additionally, stream simulation structures are less susceptible to damage by high flows and debris blockage because they do no constrict the channel until flows substantially exceed bankfull flow conditions.