Ecological Processes –
Hydrology
and Sediment
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.
