Abstract: Throughout North America stream simulation and embedded / recessed culverts are used to facilitate the passage of aquatic organisms thru road stream crossings. Although the concept of naturelike streambeds inside these structures is widely embraced, the design methodology and construction practice varies widely between state and federal agencies. Much debate has occurred with one specific requirement, whether or not to place stream bed material inside these structures. Not placing bed material inside structures assumes on going sediment transport processes will fill the structure’s interior with streambed materials. This is assumed to be a cost saving measure from both a design and implementation standpoint. Recent research, monitoring, and historic installations provide evidence that not placing streambed material can produce deleterious effects to the stream and aquatic habitat, cause low flow barriers, may not retain bed material, and potentially cause long term structural failure. Conversely in some channel types or site condition infilling may not be necessary and produce satisfactory results. Casual mechanism of success and failure, stream impacts, and design considerations will be discussed along with recommendations for site specific conditions where infilling structures is required or allowing structures to fill naturally would be successful.