Depending on the degree to which road crossings constrict the river or stream channel, crossing structures can change the hydrology and hydraulics of the system by increasing the detention time of water upstream of the crossing. The more crossings on a particular river or stream the greater the potential impact on hydrology. If the changes in hydrology are significant enough, it can potentially result in changes in sediment transport and natural scouring of the channel during storm events or spring floods.
Large woody debris is an important component of stream ecosystems. Large logs in the stream can dam up water or create plunge pools on the downstream side of the log. Accumulations of woody debris can change the local hydraulics of the stream, scouring some areas and depositing the material in other places. Where woody debris forms dams across the stream they can create large and relatively deep pools. These habitat features (woody debris, scour holes, pools, deposited gravel) are important characteristics of habitat. However, they are not permanent features and woody debris will eventually break up or move downstream. When crossing structures restrict the ability of woody debris to pass downstream, road crossings can inhibit or prevent the formation of natural debris dams and deflectors that are important habitat features for fish and wildlife, and play an important role in shaping channel characteristics.
The movement of organisms within rivers and streams is an important ecological process that can be significantly affected by road crossings. If not properly designed, road crossings can block animal movements, delay migration (a process made worse where there are many crossings), and cause physiological stress as animals expend energy passing both natural and artificial obstacles. Delays in movement can also result in overlap of individuals that typically occupy different stream reaches. For example, culverts often concentrate migrating fish in large pools at their outlets. These pools often provide resident fish habitat, and these fish can experience increased predation or competition from upstream migrants when this overlap occurs. Increased susceptibility to fishing pressure and stress associated with over-crowding can also occur when fish movements are delayed at road crossings.
Interrelationships
Habitat is a combination of physical and biological characteristics of an area (or areas) essential for meeting the food and other metabolic needs, shelter, breeding, and over-wintering requirements of a particular species. For some species habitat can be as small as individual rocks or pebbles in the streambed. For others it can include many miles of rivers, streams, floodplains, wetlands, and ocean.
At any scale – from individual rocks in a streambed to particular habitat types (riffles, pools, cascades) to an entire river system – the particular characteristics of an area will determine what species are likely to be present. The tendency of areas to form structurally and functionally distinct portions of the landscape (e.g. riffles, pools, runs, floodplains, headwater streams, tidal rivers) means that organisms that inhabit these areas often form distinct assemblages of species called communities. These communities of organisms plus the physical environment they inhabit are what constitute ecosystems.
Natural communities are more than just collections of organisms. Species that make up communities are interconnected by a variety of ecological relationships, such as nutrient cycling and energy flow, predator-prey relationships, competition, and interdependency. For example, a single stream reach may support a variety of salmonid fish species competing with each other for food and appropriate habitat. Diverse communities of invertebrates are essential for providing a food base throughout the year for fish. Disease organisms, parasites, or predators that differentially affect species will affect the balance of competition among these fish.
The presence or absence of fish can affect whether other species are able to utilize river or stream habitats. Many amphibians require aquatic habitats that are free of fish in order to successfully breed. These species may utilize floodplain pools or intermittent sections of streams as long as fish are not regularly present. On the other hand, numerous species of North American freshwater mussels require specific fish hosts in order to complete reproduction. Larval stages of these mussels, known as glochidia, attach themselves to the gills or fins of host fish (or in one case, host salamanders), a process essential for proper development and dispersal. The nature of these interdependencies is such that freshwater mussels are unable to occupy otherwise appropriate habitat if their particular fish hosts are not present.
Hydrology and Sediment Transport
As the defining feature of aquatic systems, the amount, distribution, movement and timing of water is a critical factor shaping aquatic communities. Many organisms time their life cycles or reproduction to take advantage of or avoid specific hydrological conditions. Seasonal changes in stream discharge provide predictable periods of high and low flow affecting a range of other habitat conditions (velocity, turbulence, turbidity, temperature, oxygen levels, and the spatial distribution of rapids, riffles, runs, pools and backwaters). Periodic flood events provide riverine species with access to floodplains and pools for breeding, feeding or refuge from unfavorable conditions in the main channel.
Flowing waters also transport sediment downstream, changing the substrate characteristics of areas contributing and receiving the material. Over time in stable stream segments, sediment lost downstream is generally replaced by material transported from farther upstream.
Water, sediment, riparian vegetation and materials making up the stream bed and banks interact to determine channel characteristics. Bankfull discharge is the amount of water that just fills the stream channel and where additional water would result in a rapid widening of the stream or overflow into the floodplain. Bankfull events, which typically occur every 1½ or 2 years, are generally considered to be of primary importance for determining channel form. Less frequent flood events are responsible for large scale changes in channel alignment (meandering, meander cut off) the scouring and filling of pools, and the transport of large material (cobble, boulders and large woody debris) within a stream.
Streams are dynamic systems that change over time in response to natural stream processes including rare flood events, as well as changes in watershed characteristics that can affect the hydrology, sedimentation rates, and riparian vegetation associated with streams. By contrast, road-stream crossing structures are rigid and may prevent these dynamic processes from operating. Road-stream crossings are often narrower than the stream channel and result in higher velocities and changes in stream hydraulics that can affect sediment transport and deposition both upstream and downstream of the structure.
Woody Debris
Woody debris within streams and rivers are both a habitat feature for many species and factors that can significantly change the physical and biological characteristics of streams. Debris dams or partial dams (deflectors) can create pools and scour holes, and change patterns of sediment deposition within the stream channel.
Organic debris ranging from leaf litter to large tree trunks provide substrates and shelter for invertebrates. In addition to their own inherent value, invertebrates are important components of river and stream ecosystem due to their role in nutrient processing and food chain support. Accumulations of debris (branches and tree trunks) provide underwater shelter for fish and wildlife, basking sites for turtles and snakes, and foraging areas for amphibians, minks, raccoons, and other predators.
Large woody debris (large branches and tree trunks) and debris dams are also important for their role in altering stream hydraulics and influencing local instream habitat conditions. Woody debris can dam up stream sections creating pools in areas that would otherwise lack them. Water plunging over woody debris may create plunge pools. Woody debris may also divert or constrict flows, changing patterns of scour and deposition and creating deep holes and fresh gravel beds that serve as important habitat for fish and wildlife.
Road-stream crossings tend to block the downstream movement of woody debris with the potential to create blockages at the structure entrance and reducing the amount of debris in downstream sections. Removal of woody debris from stream channels in order to help safeguard undersized structures robs the stream of these important elements of instream habitat.
Disturbance
Natural disturbances such as floods, droughts, wind throws, landslides, ice formation and scour can disrupt more regular cycles of stream flow, sediment transport, and the amount and distribution of woody debris. However, even these disturbances are part of larger patterns of physical and biological change that help define aquatic ecosystems. In fact, these more extreme events are generally responsible for defining large-scale channel characteristics.
Natural disturbances have been part of the mix of factors shaping and reshaping river and stream systems for thousands of years. Within unaltered watersheds with intact floodplains and channel characteristics, natural disturbances are likely to cause changes in rivers and streams that fall within the historic range of variability for those systems.