Channel engineering here refers to dredging and straightening channels, removing naturally occurring obstacles such as logjams and wood rafts, and stabilizing channel banks. These modifications are undertaken for a variety of reasons, including navigation, flood control, waste disposal, and protection of property and infrastructure that would be affected by lateral movement of a channel.

Navigation in the contiguous United States began long before people of European descent arrived. Many of the people living in the country at first European contact became famous for their skill in constructing and using boats such as birchbark canoes. Early European explorers and settlers used canoes, bullboats, bateau, flatboats, keelboats, and other forms of watercraft, but the modification of the country’s river network to enhance navigation dates to the era of commercial steamboat travel (circa 1829-1890). Steamboats were notoriously dangerous, causing many deaths when the boiler providing steam power exploded. Fatal steamboat accidents also resulted from grounding on sandbars or damage to the boat from snags and sawyers – large pieces of dead wood partly buried in the streambed and protruding up into the water column. Massive, naturally occurring accumulations of wood such as the Great Raft on Louisiana’s Red River also blocked steamboat passage. Steamboats were nonetheless the most inexpensive and convenient way to transport large quantities of goods and people until the country’s railroad network developed. Consequently, local jurisdictions and the federal government systematically removed large wood from channels and dredged channels to facilitate steamboat traffic. The first snagboat, designed to remove submerged or partially submerged logs from a river channel, was built in 1829 to remove logs from the Ohio and Mississippi Rivers. Even the limited records kept by the federal government indicate that 1.5 million snags were removed from 30 rivers between 1867 and 1912.

Naturally occurring large wood provides many physical and ecological benefits in channels and on floodplains. By partly obstructing the flow and creating hydraulic roughness, individual wood pieces or logjams create pools that provide fish habitat and enhance overbank flows that hydrologically connect channels and floodplains. Large wood also drives hyporheic exchange, as described above in connection with beaver dams. Large wood enhances habitat diversity within a channel, supporting the entire aquatic food web, from microbes and attached algae to larval insects and fish. Large wood on floodplains provides habitat for insects, amphibians, reptiles, birds, and small mammals, and the decay of the wood releases nutrients to fungi and a variety of plants.

The once-extensive forest cover of the contiguous United States resulted in abundant downed wood within river corridors. Persistent accumulations of wood that stretched for kilometers along channels are described in historic documents from the northeastern, southeastern, Gulf Coast, Pacific Northwest, and upper midwestern regions of the contiguous US. By obstructing downstream flow, these wood rafts enhanced overbank flows that supported extensive floodplain wetlands. The wood rafts also enhanced channel avulsion, which occurs when a blocked channel abruptly finds another course across the floodplain. Avulsion helped to create and maintain multi-channel rivers and secondary channels that provided diverse floodplain and channel habitat.

Collins, B.D., D.R. Montgomery, and A.D. Haas. 2002. Historical changes in the distribution and functions of large wood in Puget Lowland rivers. Canadian Journal of Fisheries and Aquatic Sciences, 59, 66-76.

Collins, B.D., D.R. Montgomery, K.L. Fetherston, and T.B. Abbe. 2012. The floodplain large-wood cycle hypothesis: a mechanism for the physical and biotic structuring of temperate forested alluvial valleys in the North Pacific coastal ecoregion. Geomorphology, 139-140, 460-470.

Harmon, M.E., J.F. Franklin, F.J. Swanson, et al. 1986. Ecology of coarse woody debris in temperate ecosystems. Advances in Ecological Research, 15, 133-302.

Triska, F.J. 1984. Role of wood debris in modifying channel geomorphology and riparian areas of a large lowland river under pristine conditions: a historical case study. Verhandlungen Internationale Vereinigung Limnologie, 22, 1876-1892.

Wohl, E. 2014. A legacy of absence: wood removal in US rivers. Progress in Physical Geography, 38, 637-663.

Wohl, E., N. Kramer, V. Ruiz-Villanueva, D.N. Scott, F. Comiti, et al. 2019. The natural wood regime in rivers. BioScience, 69, 259-273.

Dredging and channelization involve deepening and, commonly, straightening a channel in order to increase conveyance (the ability to convey water swiftly downstream), maintain a minimum depth for boat traffic, and/or limit overbank flooding. As with other forms of river modification, dredging and channelization under the direction of the federal government spread rapidly during the steamboat era and then continued in support of a national navigation network used by barges carrying high-volume, low-value materials such as grain. The 1824 Rivers and Harbors Bill authorized the removal of sandbars and wood along the Mississippi and Ohio Rivers, but the federal government became much more intensively involved in channel engineering after the 1936 Flood Control Act authorized the US Army Corps of Engineers to cooperate with local governments in dredging and channelizing rivers. Federally financed dredging and channelization reached an apogee during the 1960s, when even very small channels in croplands were routinely channelized for flood control.

Dredging and straightening a channel does increase conveyance, but also removes many of the irregularities (expansions and constrictions, bedforms, large wood, bends) that help to dissipate flow energy. Consequently, floods are more likely to erode the channel boundaries. This erosion results in increased sediment deposition in downstream portions of a river corridor beyond the channelization, as well as destroying habitat within the channelization portion of river. Although fewer streams are now channelized, navigational waterways continue to be routinely dredged to maintain a minimum depth for boat traffic. Construction of locks and dams to maintain water depth along rivers used for navigation can also help to maintain ideal water depths for boat traffic, but typically creates side effects such as blocking fish movement along a channel.

Committee on Government Operations. 1973. What Federally Financed Draglines and Bulldozers do to our Nation’s Streams. Fifth report by the Committee on Government Operations together with additional views, US Government Printing Office, Washington, DC.

Scarnecchia, D.L. 1988. The importance of streamlining in influencing fish community structure in channelized and unchannelized reaches of a prairie stream. Regulated Rivers: Research and Management, 2, 155-166.

Schoof, R. 1980. Environmental impact of channel modification. Water Resources Bulletin, 16, 697-701.

Simpson, P.W., J.R Newman, M.A. Keirn, R.M. Matter, and P.A. Guthrie. 1982. Manual of Stream Channelization Impacts on Fish and Wildlife, FWS/OBS-82/24.

Wohl, E. 2004. Disconnected Rivers: Linking Rivers to Landscapes. Yale University Press, New Haven, CT.

Bank stabilization involves artificially strengthening a streambank to keep it from eroding and to limit sideways movement by the channel. Bank stabilization can be undertaken in connection with dredging and channelization, or by itself. Fully functional natural rivers continually erode some parts of their banks while depositing sediment along other parts. This exchange of sediment helps to dissipate flow energy and creates habitat diversity. A riverside forest along a channel with no bank erosion, for example, will gradually age and die. Local bank erosion kills some mature trees, but also provides germination sites and helps to maintain age- and species-diversity of the forest.

Bank stabilization is also commonly an expensive and ongoing process along any particular portion of a river. Where a channel is heavily stabilized along both banks, enhanced bank erosion and channel widening typically occur immediately downstream from where the bank stabilization ends because the so-called ‘hungry waters’ that have been deprived of sediment through the stabilized reach have excess energy to erode the banks.

Florsheim, J.L., J.F. Mount, and A. Chin. 2008. Bank erosion as a desirable attribute of rivers. BioScience, 58, 519-529.