Channelization is simply the act of making a channel. The term is most often used when talking about waterways, although it has also been co-opted into use in the field of traffic engineering1.
Natural watercourses tend to meander, and will often spread out over the land in large deltas or marshes. These spreads can be very large, as evidenced by the Mississippi River delta. This is a great thing for genetic diversity, management of a healthy water cycle, and fertilizing the local floodplain, but it isn't so good for towns, cities, and farms. It has long been a goal of developers to drain the swamps and straighten the rivers, making them nice and neat and civilized.
Generally, channelization involves selecting the central or most desirable channel of the waterway and dredging it to make it deeper and/or wider. Smaller side channels and wetlands may be filled, and if the waterway meanders, new channels may be dug to straighten it out. Banks and levees may also be built up to contain the waterway in times of flooding. In some cases, particularly when the river is travelling through a developed area, the channel may be lined with concrete to discourage erosion and speed flow. It is not uncommon for large channelization projects to reclaim hundreds of acres from floodplains and wetlands.
The basic goals of channelization are generally: to make rivers more navigable; to contain the river and make surrounding lands more manageable; to control flooding; and to control erosion.
Channelization does tend to make rivers and waterways more conductive to river travel and shoreline development, and in draining wetlands. These are the primary reasons why channelization has been so popular for so many decades. However, it has not been so successful at meeting the other goals, and it always has negative effects on the pre-existing ecology, often with disastrous environmental effects.
Surprisingly, channelization does not always do a good job of controlling flooding. This is largely because when engineering a major channel the engineers have to put a cost on the management of various flood levels. Every channel should be designed to safely manage yearly fluctuations in water levels, which may peak during the local rainy season or during spring thaws in the water system's headwaters. Statistically, some years will have greater flow than others; there are some levels of flooding that will be expected to encountered every decade, others that are so rare that they are encountered every hundred years (on average). Few engineers are paid to design a channel that can manage a flood that is expected to come about only once in a hundred years, so when that flood comes along, the river jumps its boundaries and causes excessive destruction. Changes in a river's flow may also increase flooding up- or down-stream from the channelization2.
Often, the act of channelization and the sense of security that comes along with it can cause this flooding to be worse than it would otherwise be. Hurricane Katrina's disastrous effects on New Orleans may be the most famous example of a developed area being overly dependent on heavy channelization of surrounding waterways; an unusually large storm surge entering the Mississippi River delta caused channels to breach the levees surrounding the city in multiple locations, resulting in hundreds of deaths and incalculable economic damage. In this case, as in many others, the structures meant to prevent flooding also prevented flood waters from draining once they were breached.
While channelization has generally done fairly well in preventing erosion, when specifically designed for this purpose, the faster water flow has the potential to increase erosion if banks are not reinforced. Sudden decreases in water flow also encourage deposits of sediment, which can cause channels to slowly fill back in.
Perhaps most importantly, channelization has had a major impact on the environment. It has been estimated that in the US (excluding Alaska), the amount of riparian habitat has decreased from 121 million acres to 23 million acres3. This means reduced habitat for many species; increase runoff of nutrients and pollutants into lakes and oceans, as these would normally be filtered through floodplains and marshes; increased volume of water into lakes and oceans, as water that would be absorbed into aquifers and lost to transpiration is carried quickly down the channel; and sharper peaks in water flow, as water that would normally take weeks or months to pass through wetlands is passed in a matter of days.
While channelization in various forms is still common practice, it is becoming more common to look for alternatives, and to consider wetlands useful rather than pointless wastes of land. Projects to straighten, deepen, and levee waterways are now undertaken with considerably more caution than they have been in the past, and agencies such as the Conservation Reserve Program, the Wetland Reserve Program, and the Stewardship Incentive Program work to encourage limited and judicious use of channelization projects.
Footnotes & References:
1. In the case of traffic engineering and city planning, channelization refers to the use of secondary or specifically constructed roads to take some of the traffic flow off of main roads.
2. Increased flow and increased peaks of flow cause flooding downstream. Upstream flooding may be caused if entry into the new channels is limited by levees or other structures.
3. Brinson, M.M., B.L. Swift, R.C. Plantico, and J.S. Barclay. 1981. Riparian ecosystems: Their ecology and status. FWS/OBS-81/17. U.S. Fish and Wildlife Service. Kearneysville, W.V. 154 pp.