Disconnected Rivers Linking Rivers to Landscapes
By ELLEN E. WOHL
YALE UNIVERSITY PRESS Copyright © 2004 Yale University
All right reserved. ISBN: 978-0-300-10332-8
Chapter One Why Should We Care About Rivers?
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.
They 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 hillslopes, and rivers become broad and shallow. Where people mine precious metals from hills or build electronics 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.
River valley sediments record all the changes in a river's drainage basin over thousands of years. The river itself records the most recent changes, steepening its course as it crosses the furthest sediments deposited by a glacier now melted, or dammed where farmers in the 1950s wanted water storage.
The organisms living in and along rivers alsoreflect 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 twenty-first 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. Where dammed water released from the bottom of a reservoir creates a cold, clear flow, introduced trout thrive, out-competing the native fish adapted to the warm, sediment-laden waters present before the dam was built. And where a dam blocks native fish returning from the sea to spawn, these fish are no longer present at the headwaters of the river.
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. Organisms living in and along a river also have an evolutionary history, and the unique evolutionary lineages present in different river drainages provide clues to the history of isolation or integration of each drainage.
We owe rivers the respect due to any source of information that helps us to understand our history, and so to understand ourselves. But rivers are also our lifelines. They provide us with the water we drink, the water that helps our crops to grow, and the water that fuels or cools our industries. Water is a universal solvent and is used at some stage in the manufacture of every product we consume. Rivers transport our wastes, and to some extent transform them. If not for this self-purifying function of rivers, many of our estuaries and deltas would be even more polluted. Rivers transport our goods, generate our power, and sustain our recreation. The condition of our rivers, more than any other natural resource, reflects our attitudes toward the world around us, and ultimately our attitudes toward ourselves. The society that does not protect its rivers destroys its own lifelines.
This book draws a connection between lack of respect for rivers and lack of understanding of rivers. We in the United States have not fully appreciated the vital functions that rivers perform.
Human beings have used the natural systems of America as resources since the first people reached this continent. Such use reaches unsustainable levels whenever people deplete a physical resource to the point that the resource is effectively no longer available to them, or whenever they deplete a biological resource to the point that a species can no longer sustain itself or perform its ecological functions. Unsustainable irrigation practices led to the salinization of Hohokam agricultural fields in the southwestern United States by A.D. 1300. Unsustainable hunting, trapping, and fishing by European Americans led to the extinction or near-extinction of beaver in the eastern United States during the 1600s, the bison by 1880, the passenger pigeon in 1914, and commercial fisheries for shad, cod, sturgeon, and other species during the later nineteenth and twentieth centuries. Unsustainable logging practices altered forest ecology and composition in a manner that will require centuries to overcome in regions as diverse as the southern Appalachians (1900s-1930s), the Colorado Rockies (1860s-1890s), and the California coast ranges (1950s). Unsustainable river flow regulation and diversion led to the endangerment of native fish, massive loss of riverside vegetation, erosion of archeological sites, and degraded water quality along the Colorado River as well as to the collapse of salmon populations in the Pacific Northwest.
This long history of resource destruction is partially offset by a developing vision of resource conservation. The environmental movement in the United States has emphasized the conservation of natural resources and the reservation of public lands since the late nineteenth century, when leaders such as John Muir, Gifford Pinchot, and Theodore Roosevelt persuaded the federal government to designate the first forest reserves and national parks. Public support for conservation grew during the succeeding century, along with concerns about how human activities might be affecting the world around us. Development of the modern conceptual framework of ecology during the 1960s and 1970s emphasized that the environmental health of public lands must be assessed in terms of the physical habitats that support communities of interdependent species. With increasing concern over the number of species becoming endangered or extinct, Americans are realizing how closely the species present in any community are linked to the physical landscape and to one another by numerous chemical and physical exchanges. We cannot save an endangered species of trout without also saving the river and floodplain habitat in which that trout evolved, as well as the plants and insects that form the food web in which that trout exists.
At the same time that the federal government reserved public lands, it also invested in land reclamation and engineering, building dams to irrigate agricultural lands and control floods and dredging and straightening rivers that were then confined within levees. While the government created legislation guaranteeing the quality of air and water and protecting endangered species, it also subsidized road building, timber harvesting, mining, and grazing on public lands.
The United States thus has two competing traditions. One tradition emphasizes individual and corporate freedom to optimize short-term profits, with economic growth and increased standards of living based on the excessive exploitation of natural resources. The other tradition emphasizes resource conservation and environmental protection, an interest in natural history, and expectations of outdoor recreation and public access to wilderness areas. These competing traditions together shape the understanding and use of rivers in the United States.
Despite the history of public awareness of environmental issues in this country, many people remain unaware of how substantially human activities have altered rivers across the nation. Human activities affect the movement of both water and sediment along a river-the river's form. Human activities also alter the river's ability to provide habitat and nutrients for diverse species-the river's function. The distinction between form and function is important because it governs public perceptions of rivers.
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 its banks or fish in its pools. These expectations of a healthy river's appearance may be misleading in that they ignore loss of function. However, it is difficult to assess a river's function 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 processes of nutrient exchange to which the local in-channel and floodplain communities of plants and animals are adapted.
The channel bed of a natural river, for example, is unlikely to be of uniform depth or material for more than a few tens of yards downstream. Most channels have alternating deep pools and shallow riffles. The riffles have coarser sediment and faster and shallower water. Species of aquatic plants, insects, and fish adapted to rapid, shallow water favor riffles, whereas a few yards upstream a different community of species will inhabit the deeper waters of a pool. Because these differences are not readily apparent to an observer, a river with severely impaired function may appear to be healthy. A river with clear water and stable banks supporting a few mature cottonwood trees meets many people's expectations of a picturesque river. But clearing of wood from the river channel may have destroyed the pools and riffles, changing a diverse in-channel habitat supporting numerous species to a largely uniform channel supporting only a few species. Flow regulation associated with upstream dams may have altered the river's flow such that the banks are more stable and cottonwoods, which germinate on freshly deposited sandbars, are no longer reproducing. Changes in river form have led to an impoverishment of river function and a decrease in biological diversity. If we do not understand how a natural river would really appear or how it would function, however, we will not recognize when a river environment has been altered.
We cannot save trout without saving their river and floodplain habitats. We cannot save river and floodplain habitats-and the plants and insects of the trout's food web-if we do not also maintain the processes controlling water and sediment entering the river corridor from the surrounding hillslopes and uplands. They go hand in hand. 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 hillslopes adjacent to the river during thousands of years of weathering of the underlying bedrock; the plant communities growing on those 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. We increasingly have discovered that it is enormously expensive and difficult to artificially re-create these processes-to pass salmon through a dam with fish ladders, for example, or to stabilize and revegetate clearcut hillslopes. River corridors function most fully and effectively when they remain connected to the total environment.
Today, the American people are being asked to make decisions regarding rivers from the national level (in relation to the Clean Water Act, the Wild and Scenic Rivers Act, the National Floodplain Insurance Act, the Endangered Species Act, and other federal legislation) to the local level (in relation to wastewater management, nonpoint source pollution, in-stream flow, flood hazards and community zoning, open space and greenbelts, recreation, and dam relicensing and removal). Most people have an instinctive appreciation for flowing water and look to rivers as a source of recreational and esthetic enjoyment. But others still regard rivers as mere conduits for the transport of commodities and wastes or as natural hazards that must be controlled. These conflicting ways of seeing rivers and hence demands on river resources only intensify as both global population and U.S. population and resource use continue to grow. If we are to make informed decisions regarding rivers, it is important that we learn to think of rivers in terms of both form and function. This requires that we look beyond the obvious physical characteristics of a river and think of it as an extensive ecosystem interdependent with and connected to the surrounding floodplains and drainage basin.
Chapter Two American Rivers
The rivers of the United States are as diverse as the country's people. The rivers meander slowly across marshy plains hazy with heat and humidity. They rush down steep, rocky gorges fed by the melting ice of glaciers. They flow hidden beneath the ground in limestone caves, or they flow only after a thunderstorm has abruptly saturated the desert's surface.
Animals adjust themselves to this diversity. Silvery salmon swim relentlessly up clear, cold waters to lay their eggs among gravels eroded from the jagged Sawtooth Mountains of Idaho. Catfish with barbels sensitive to subtle movements in murky water wait beneath overhanging banks of clay in the lowlands of Louisiana. Mallard ducks call to one another as they land on the gray-green waters of the upper Mississippi. Beavers work steadily with slender branches and saplings, anchoring them among cobbles pushed up into a ridge, and overnight a small mountain stream in Utah is dammed.
Starting at least twelve thousand years ago, humans also adjusted to this rich diversity of river environments and shaped the rivers to their own needs. Human impacts to rivers depend in part on the characteristics of each river ecosystem, and these characteristics reflect the geology, climate, and history of a river basin.
The Physical River System
A river channel conveys water and sediment downstream. Imagine two hypothetical rivers: one flowing from the Rocky Mountains in the southwestern United States, the other flowing from the southern Appalachians. The contrasts between these rivers illustrate how geology and climate interact to govern the amount of water and sediment supplied to a river, and how the water and sediment supply control river form and process. Both rivers begin in steep terrain where a long history of uplift contorted the bedrock into mountains.
Faulting and the intrusion of large masses of molten rock into the overlying crust during millions of years created the Rockies. Alpine glaciers ground away at their upper valleys until about ten thousand years ago. As the ice widened the valleys, it also deposited huge boulders along them and, when it melted, sent much higher volumes of water and sediment down the rivers. The granite underlying the drainage basin resists weathering and erosion in the relatively dry climate present since the retreat of the glaciers. The tough rock slowly weathers to cobbles and gravel that form steep slopes below the bedrock cliffs. Where softer rock such as shale is present, weathering produces silt and clay that form rounded, gentle slopes.
Excerpted from Disconnected Rivers by ELLEN E. WOHL Copyright © 2004 by Yale University. Excerpted by permission.
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