Disconnected Rivers: A Geologist Explains the Consequences of Reshaping Rivers

from World Rivers Review

The book "Disconnected Rivers" by Ellen Wohl (Yale University Press, 2004) reveals how human activities have impoverished US rivers and impaired the connections between rivers and other ecosystems. The following excerpts describe the natural interactions that make for a healthy river, and an effort to restore the channelized Kissimmee River in Florida.

Rivers reflect a continent’s history. Where forces far beneath the Earth’s crust force up mountain ranges, rivers flow swift and cold down steep, boulder-strewn channels. Where the Earth is still, rivers meander broadly, depositing thick plains of sand, silt, and clay.

Rivers also reflect a people’s history. Where people clear the forests for agriculture, river valleys retain sediments, recording the transitional period when the soil washes down from the hill slopes, and rivers become broad and shallow. Where people mine metals from hills or build factories, river valley sediments contain the toxic by-products of these activities. People build canals, roads, and railroads along river corridors, following river passages through dense forests or steep mountains.

The organisms living in and along rivers also reflect this history. Along a river downstream from a site where mining occurred in the 1890s, there are fewer individuals and species of aquatic insects and fish in the 21st century because toxic metals still leach from the mining site. Where a river repeatedly shifted its course back and forth across the valley bottom during floods spread across 200 years, cottonwood seedlings have sprung up on each new sandbar created by a flood. Now the river has groves of cottonwoods aged 10, 40, 80, and 175 years, and these trees map the changes in the river’s course.

The physical forms of rivers and river ecosystems are our historical archives, yet these archives are challenging to interpret. Gaps may be present in the physical record where sediments deposited during an earlier period of river history were subsequently eroded. Because of the gaps we can seldom decipher a complete and continuous record of a river’s history. But by assembling the records from many rivers we can piece together regional and continental syntheses of history.

The form or physical appearance of a river can be readily perceived. People commonly expect a “healthy” river to be pretty; to have clear water, stable banks and bed, and perhaps a fringe of trees along the banks or fish in the pools. These expectations of a healthy river’s appearance may be misleading in that they ignore loss of function. It is difficult to assess a river’s function, however, with only a casual examination. River channels are fundamentally conduits for water and sediment, but the specific processes of water and sediment movement vary widely among channels. These processes create unique habitats and patterns of nutrient exchange to which the local in-channel and floodplain communities of plants and animals are adapted.

A functional river ecosystem is connected to everything around it: the atmospheric and oceanic circulation patterns that control precipitation over the drainage basin; the soils developed on the slopes adjacent to the river during thousands of years of weathering of the underlying bedrock; the plant communities growing on these soils, and the animals that pollinate and consume the plants; the processes by which precipitation filters down to the groundwater and raises or lowers the water table that is intimately connected to most streams, and on and on. By altering our river systems we have, in many cases, severed these vital connections. Dams interrupt the upstream-downstream passage of fish, the downstream flow of seeds that replenish riverside forests, and the downstream movement of water and sediment. Timber harvests short-circuit the gradual downslope flow of rainwater below the ground, instead sending masses of water and sediment quickly into nearby rivers. Artificial levees keep young fish from the rich nursery habitats created by warm, shallow waters spreading across a floodplain during high flows and prevent the pulse of nutrients returned to the channel as floodwaters recede. Disconnected rivers become impoverished in form and function because the processes maintaining form and function no longer operate.

Rivers in Chains

Channelization is the widening, deepening, clearing, and/or straightening of river channels. Such activities are undertaken for several purposes. Channelization drains wetlands by speeding the passage of water through the wetlands and lowering the groundwater table. It reduces flooding of adjacent lands by increasing the river’s capacity to transport flood flows, or enhances navigation by increasing the natural depth of larger rivers. It also controls erosion by substituting artificial canals for gullies or other eroding natural channels. Individuals or local communities have undertaken channelization for two centuries in the US, but the practice became much more extensive under the supervision of the federal government during the 1940s. More than 34,000 miles of waterways were channelized by the Army Corps of Engineers and the Soil Conservation Service after 1940.

It became apparent within 30 years that channelization had some unanticipated consequences. A 1973 congressional report noted that most of the open ditches constructed before 1940 to drain wetlands were poorly engineered, poorly maintained, and poorly designed in relation to their larger watersheds. Consequently, the local entities constructing ditches solved the flood problem by dumping it downstream. The federally financed channelization projects of the 1940s to 1970s were not much better. The 1973 report on these more recent projects is damning. The report emphasizes that “inadequate consideration was being given to the adverse environmental effects of channelization. Indeed, there is considerable evidence that little was known about these effects and, even more disturbing, little was done to ascertain them.”

Scientists have demonstrated the adverse environmental effects of channelization to be many and various. In addition to loss of upland soil, adverse lowland effects include impacts to wetlands, riverside vegetation, river form and flood flow, and aquatic organisms.

Drainage of wetlands lowers the local groundwater table, changing the water cycle and availability of nutrients for wetland plants. This eliminates or reduces the number and diversity of plant and animal species living in and using the wetlands. As the water table declines, the water-holding capacity and the capacity for groundwater recharge also decline.

Periodic flooding and lateral channel movement are natural disturbances that create spatial variability in bottomland forests. Stabilizing a river reduces variability through time and across the bottomland. Eventually, the bottomland forest shifts toward a homogeneous community of species less tolerant of flooding, which occupied the outer floodplain before channelization.

Direct cutting of riverside trees during channelization eliminates shading and the input of organic matter such as leaves and twigs to streams. With the trees and their binding roots gone, the streambanks are more susceptible to erosion. Flow velocity is not as effectively reduced along the now-smooth streambanks, and sediment is less likely to be deposited in natural levees. Cutting of bottomland hardwoods eliminates vital habitat for many animals and can increase nutrient and sediment concentrations in adjacent stream channels.

The Kissimmee

The Kissimmee River of Florida, now the center of a massive restoration effort, further illustrates the consequences of channelization and the steps necessary to rehabilitate channelized streams.

Florida is flat. Along the course of the Kissimmee River in southern Florida, the adjacent “uplands” are only 6 to 10 feet higher than the floodplain. The Kissimmee River once meandered in broad arcs across this floodplain, dropping 6 to 9 feet for every 100,000 feet traveled along its 100-mile-long drainage basin. Native Americans named the Kissimmee for its “winding waters.”

From the air, the Kissimmee drainage basin looks as though someone had thrown a handful of pebbles through a layer of brilliant green pondweed, leaving many little clear holes to the water beneath. These holes are karst lakes, for the “pond” beneath the green of the basin is a vast layer of carbonate rocks honeycombed with water-filled caves and sinkholes.

Kissimee River, Florida (Brent Anderson/SFWMD)

Once the carbonate rocks of southern Florida lost the protection of the ocean in which they formed, the rocks slowly began to dissolve. Most rainwater is slightly acidic, and it reacts with carbonate rocks, dissolving the calcium from the rock and carrying the calcium away in solution, leaving sinkholes. Water fills the sinkholes to create lakes and spills across the lowlands to form rivers and adjacent wetlands. Before European Americans intervened, the different components of this giant sponge were subtly but intricately connected. Rain that fell on the interior was carefully used, soaked up, and filtered through a huge landscape and passed on from organism to organism, nourishing the rivers, lakes, prairies, and wetlands. Black, organic-rich water drained slowly into Lake Kissimmee in the middle of Florida and then down into the Kissimmee River.

During wet periods the waters flowed south in a broad band, inundating a floodplain up to three miles wide. Eventually, the waters coalesced into Lake Okeechobee (“big water” in Seminole) and then spilled out once more to filter down through the Everglades in broad sheets. Rains during the summer and autumn covered the floodplain for 3 to 9 months each year, and periodically for the entire year. This flooding supported an annual invertebrate production on the floodplain up to one hundred times greater than the production in the stream channel. As water levels declined each year, the invertebrates were carried into the river, where they served as food for many other animals. The life cycles of water birds and fish were also linked to flooding. Wood storks ate fish concentrated at water holes during the winter, and snail kites fed on apple snails whose egg laying was tied to seasonal water fluctuations. Fish used the floodplain habitat for spawning and nursery areas. Years with a smooth increase in water level, and with large floods that lasted a long time, were good for the 35 species of fish in the region.

Human population and infrastructure began to increase more rapidly in the Kissimmee River drainage basin after World War II. A major hurricane in 1947 resulted in extensive flooding and property damage, and the Army Corps of Engineers stepped in. Congress authorized the Kissimmee River Flood Control Project in 1954. Between 1962 and 1971, the corps dredged a trapezoidal canal from Lake Kissimmee to Lake Okeechobee. The canal is about ten times the size of the natural channel, with six water-control structures that regulate water levels and flow, creating a reservoir upstream of each structure. In most areas of the world, roads go downhill into a valley to cross a river. In southern Florida, the rivers are walled within levees that sit above the surrounding lands, and roads rise up to cross a river.

The Kissimmee River Flood Control Project did alleviate flooding, but it also largely destroyed the floodplain ecosystem. Dredge material from the canal buried 6,900 acres of floodplain wetlands. Another 35,000 acres were altered by the loss of seasonal flooding. Remaining segments of the natural channel carried little or no flow. As several inches of organic muck accumulated over the sandy bottoms of these channel segments, the stagnant water became anoxic, with little oxygen available to aquatic plants and animals. Levels of phosphorus entering Lake Okeechobee increased from 100 to 500 tons per year as dairy, citrus, ranching, and sugarcane operations spread through the former wetlands. The excess nutrients created algal blooms that depleted oxygen levels in the lake waters, destroying the abundance of autumn insects that had fueled migratory birds on their way.

Almost as soon as channelization was complete in 1971, public outcry galvanized political resolve to restore the Kissimmee ecosystem. The Florida legislature passed the Kissimmee River Restoration Act in 1976. Activities are now being undertaken to restore ecological integrity. Ecological integrity of the Kissimmee River is to be judged by five factors. The first of these is the energy source to fuel the lifecycle, which depends on inputs of organic matter. In other words, is there enough mass of plant material to produce the nutrient-rich muck on which all other life depends? A second factor is water quality, judged in terms of temperature, turbidity, dissolved oxygen, and other characteristics. Is the water warm and clear and oxygenated enough to support insects, fish, turtles, and alligators? Habitat quality, as judged by streambed composition, flow depth, flow velocity, and diversity, forms a third factor. Hydrology, measured as flow and variability of water flow through time, is the fourth factor. Finally, biological interactions, including competition, predation, disease, and parasitism, must be restored.

The Restoration Plan

In order to meet the flow criteria set by project scientists, program managers evaluated three plans, each of which proposed a different method to divert water from the canal back to the remnants of natural channels. Managers eventually decided to remove two of the existing six water-control structures, backfill 22 miles of canal, re-excavate portions of the river channel that were destroyed, and purchase about 70,000 acres of floodplain through the State of Florida, all at an estimated cost ranging from $280 million to $422 million. This is one component of the $7.8 billion Comprehensive Everglades Restoration Plan, a massive experiment to determine whether we can in fact undo our negative impacts. Money is not everything, but it is a crucial component of the restoration process. A National Research Council report issued in 2003 noted that inadequate funding was hampering Everglades restoration efforts.

Managers installed three weirs across a portion of the canal below Lake Kissimmee between 1984 and 1989 as a demonstration project. The weirs simulated the effects of dechannelization by diverting flow into remnant river channels and floodplains. The demonstration project had some encouraging successes. The diverted flows increased dissolved oxygen levels in the river water. The flows carried downstream the fine sediments that had accumulated in the river since channelization and restored the sandy streambed. The flows also re-contoured the uniformly flat, shallow channel into a channel with pools and sandy riffles. As the physical integrity of the river was restored, biological integrity also began to recover. Bottom-dwelling invertebrates became more numerous and diverse. Game fish became more abundant. But the demonstration project also indicated the limitations to river rehabilitation. More complete restoration of the river’s biological integrity requires the re-establishment of historical flow patterns. As cautiously summarized by the editors of a special scientific volume on the restoration effort, “With all our expertise in ecosystem restoration, it is widely recognized that it is unlikely that the full spectrum of structural and functional attributes of the system can be restored to the levels existent prior to the disturbance.” All the king’s horses and all the king’s men couldn’t put Humpty together again …

End Note

We all live among rivers. They are the sinews that bind our landscapes together. I have come to feel with increasing urgency that as we unwittingly strain or cut those sinews, we threaten the integrity of the whole environment on which we depend. We have taken rivers for granted for centuries, and we continue to do so at our peril. We cannot continue in this manner for much longer. We need to keep trying to restore rivers, but our efforts must reflect knowledge, patience, and a willingness to learn from past mistakes. Our rivers deserve nothing less.