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Streams form in the landscape when rain runs across land to its lowest point, carrying water and sediment from high elevations to downstream lakes, estuaries, and oceans. Natural streams have sequences of riffles and pools or steps and pools, meanders, and floodplains that maintain channel slope and stability. A stream and its floodplain comprise a dynamic environment where the floodplain, channel, and bed evolve through natural processes that erode, transport, sort, and deposit alluvial materials. The result is a dynamic equilibrium in which the stream maintains its dimension, pattern, and profile over time, neither degrading nor aggrading.
Land use changes in the watershed, channelization, culverts, removal of streambank vegetation, impoundments, and other activities can upset this balance. As a result, large adjustments in channel form, such as extreme bank erosion and/or incision, will happen. A new equilibrium may eventually result, but not before the associated aquatic and terrestrial environment are severely damaged. Understanding natural stream processes and applying this knowledge to stream restoration projects will help create a self-sustaining stream with maximum physical and biological potential.
Natural channel design is a method of restoring a stream by engineering changes to mimic natural conditions. This might include re-establishing meanders, planting trees in the riparian corridor, replacing woody debris in the stream for habitat, and re-connecting the channel to the floodplain. To learn more about natural channel design as an option for stream restoration in your watershed, visit the NRCS Stream Corridor Restoration Web site for several sources of information and materials for help guide your implementation planning. The North Carolina Sea Grant Program has a guide book entitled Stream Restoration: A Natural Channel Design Handbook [PDF] that may also be helpful.
Two-stage ditches [PDF] are a compromise between natural channel restoration and drainage needs. While not strictly natural channel design, two-stage (and other multi-stage variations) ditches are designed to include an in-stream floodplain in the form of a “bench” on one or both sides of the channel. Floodwaters can spread over the benches, slowing the water and dropping sediment onto the vegetated benches. Typically, drainage tiles outlet onto a bench where nitrates in tile-water are processed by vegetation and microbes growing on the bench. No dredging occurs during construction of the two-stage, so that the channel can establish equilibrium within the confines of the ditch, benefiting aquatic organisms.
Another stream restoration method includes “daylighting” of once natural or man-made streams. Daylighting of the urban buried streams is the intentional re-exposing of piped or culvert-covered streams, rivers, creeks, and storm water drainages to the open air. Daylighting buried streams can provide a variety of benefits to local governments, including reduced erosion, improved water quality, increased property values, additional aquatic and riparian habitat, and reduced flooding problems caused by under-capacity culverts.
During 2000 the Rocky Mountain Institute (RMI) in Colorado produced a 64-page report entitled Daylighting: New Life for Buried Streams. It discusses daylighting and its costs and benefits to local governments. The report documents 18 completed projects and provides information on an additional 23 projects in various stages of consideration around the country. International and two Minnesota case studies are included. An additional resource on this topic is the U.S. National Park Service brochure, Giving New Life to Streams in Rural City Centers [PDF].