# Forestry Best Management Practices Relationships with Aquatic and Riparian Fauna: A Review
## 3 Results and Discussion
### Stream Crossings  
In the absence of BMPs, both permanent [71] and temporary stream crossings have the potential to cause long-lasting effects on aquatic and riparian environments and organisms if not properly designed or maintained [72; 73; 74; 75]. However, application of BMP technology at stream crossings can help substrate heterogeneity and stream flow regimes, and retain streambank integrity [72; 76; 77; 78], all of which are important environmental characteristics for maintaining and conserving healthy aquatic and terrestrial riparian wildlife [71; 79].  
Stream crossing location and design are important aspects of forestry BMPs. Improper selection of stream crossings can cause changes in hydrology and sediment inputs, which in turn may influence population dynamics and in-stream and terrestrial habitat [72]. Maintaining stream channel morphology helps maintain ecological communities and populations of riparian species [72]. To safeguard aerial, terrestrial, and aquatic riparian wildlife, stream crossing designs need to consider river morphology, depth, velocity, stream flow, and scouring potential [80; 81; 82; 83]. These considerations are vital to maintaining healthy wildlife populations; poorly designed stream crossings increase sedimentation, cause issues with aquatic organism passage, influence population dynamics, and contribute to loss of suitable habitat [72; 76]. Legacy stream crossings installed prior to the era of BMPs may require replacement or remediation [57]. Although bridges, which usually can be installed to preserve natural channel shape, are a preferred stream crossing type, particularly over larger streams, other factors such as traffic requirements, structural loading capabilities, site features, and economic feasibility often favor use of other stream crossing types [83; 48; 84].  
Culverts typically are the structure of choice for crossings in smaller streams. Culverts that are improperly sized for the watershed can have reduced capacity to pass water and sediment or accommodate fish passage [83]. Water velocity at the exit from culverts that are undersized for the watershed can exceed the capacity of the channel and cause greater downstream scour [85]. Increased velocity and potential culvert suspension could result in habitat and population fragmentation of aquatic species by creating a barrier to aquatic organism movement upstream [73; 76; 83; 86]. Proper culvert size selection is a very basic BMP application [48]. By considering the benefits and potential impacts of stream crossing selection options at a particular site, land managers can select an appropriate stream crossing that will minimize potential ecological and environmental impacts [76; 84].  
Potentially negative effects of culverts are also contingent on other design parameters. With careful planning and knowledge of local options, selection of stream crossing types that allow adequate organism passage can be both cost-effective and ecologically compatible. Some designs, such as open-bottom culverts, can mimic natural channel shape and substrate and preserve natural hydrological attributes [76; 77; 78; 83]. Although research on the effects of stream crossings on riparian and aquatic wildlife other than fish [86; 87] are relatively limited [71; 77; 78], culvert BMPs and culvert design have been shown to influence mussels [88], crayfish [77; 78], snails [89; 90; 91; 92], and aquatic insects [93; 94; 95; 96]. The complexity of culvert material, design, and placement and potential effects on a variety of aquatic organisms warrants further investigation. Current BMP effectiveness research suggests that minimizing the numbers of crossings, placement of stream crossings at sites that minimize channel disturbances, use of appropriately sized and installed culverts, and disconnecting sources of erosion from stream crossing approaches will benefit sediment-sensitive aquatic organisms [48; 72; 83].