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Streams and Energy - Large scale

Under normal circumstances, streams exist in what is known as a dynamic equilibrium with their watersheds and immediate surroundings, called riparian zones. This does not mean that streams do not change over time, or that they are in a perfect balance. Streams are part of the slow erosion of the landscape. Over time, streams move both laterally and vertically, transporting millions of tons of rocks and soils and organic matter downstream, deepening and widening valleys along the way. In an undisturbed landscape, however, streams change gradually, moving but maintaining their basic structure and equilibrium with both the landscape and the ecosystems of which they are a part. Natural streams in undisturbed watersheds are therefore more predictable, in most cases, than disturbed streams, and tend to be self-maintaining. Streams whose equilibrium has been disrupted by changes in land use, however, lose that predictability, and instead of being assets, often become expensive liabilities to both human and natural communities. The following is written to be a basic primer for the understanding of how streams work in the large scale, and what can be expected when they or their supporting watersheds are disrupted.

The natural dynamics of a stream or river are best explained in terms of energy; both physical energy, including force, friction, temperature, and gravity, and biological energy, including food chains, nutrients, and ecosystems. Streams, like all of nature and life, exist at the boundary of chaos and order. In many ways biological life and ecosystems are designed to harness these energies, and to impose some kind of order into the chaos. In the evolution of stream dynamics, nature has used the hydrologic cycle, gravity, and simple erosion to develop a highly complex system of checks and balances, seeking entropy and dividing energies to create a staggering degree of natural diversity.

Streams and rivers are nature's mechanism for transporting both inorganic and organic materials downhill and eventually to the sea. A stream's overall energy (its speed times its volume) is in balance with the amount of material that is capable of moving downstream (its bedload). Greater energy creates more erosion and more bedload, and vice-versa. Let's first look at factors that affect the speed or velocity of water in a stream. Velocity increases with slope, of course, but natural streams are sinuous, and arranged in a fairly regular pattern of "S" shaped bends. Sinuosity in a stream assures that even though it may flow through fairly steep country, its actual slope, and therefore its velocity, is lessened. To understand this concept, imagine a ramp 2 feet high. While a straight line drawn from the top to the bottom might only be 5 feet long, a string laid out like a snake from top to bottom might be 10 or 15 feet long when straightened out. Sinuosity reduces energy by reducing the slope of the stream, reducing the effects of gravity, and reducing velocity and power of the water.

There are other factors, besides slope, that influence the velocity of water in a healthy watershed. The presence of vegetation, both near a stream and throughout its watershed, is critical to a stream's health and stability in a number of ways. When rainfall is heavy enough to cause water to run across the surface of the land, or to cause streams to overfill their banks, vegetation acts to decrease the velocity of flood and run off waters. Stems, leaves, and roots create friction that slows and breaks up water currents, reducing their power to cause damage. Vegetation also serves as a natural screen, to catch debris, organic materials, and soil particles and cause them to settle out before reaching the stream channel. This benefits the stream by reducing the inputs of silts and mud, but it also benefits the plants by providing new inputs of nutrients into floodplain soils.

A vegetated watershed and stream corridor also reduce stream energy by reducing volume. In an undisturbed watershed soil litter layers and wetlands capture and hold water, some of which subsequently infiltrates into ground water layers, and some of which is evaporated or transpired into the atmosphere. A single, healthy, floodplain tree can transpire over 200 gallons of water into the atmosphere per day. Combined with shrubs, marshland, and meadow vegetation, transpiration can remove thousands or even millions of gallons of water that would have otherwise entered the stream. A well vegetated watershed also tends to moderate how quickly rainwater enters a stream, and how long it stays. For all of the reasons described above, vegetation and undisturbed, permeable soils in a watershed cause floods to occur gradually over a long period of time, with lower peak flows and a greater time lag between a rainfall event and the high water event in the stream.

This does not mean that vegetation starves a stream of its water, however. Vegetated watersheds also prevent stream levels from becoming too low. Water that infiltrates into soil or ground water layers tends to be released slowly back into the stream channel. This keeps the stream from drying out during times of relative drought, and because the ground water is cold, it combines with the effects of shade to keep streams much cooler throughout the seasons. Vegetation in a watershed tends to preserve the stability of water in a stream, creating lower highs, higher lows, and gradual changes in water level and quality. This is in contrast to a disturbed or "flashy" stream that tends to alternate between sudden, destructive floods, and prolonged periods of low water.

Vegetation has many other important functions. In a healthy stream system, banks are relatively stable because they are held together by vegetation growing very close to or even in the water itself. Roots shield the banks from erosion and overhanging branches shade the stream and keep its waters cool. Leaves and twigs that fall or wash into the stream provide organic matter that forms the base of the aquatic food chain, and insects that fall directly into the stream (called allocthonous material) provide food for larger fishes. Tree trunks and branches in the stream and undercut banks under roots along the edges provide hiding places for fishes and other stream organisms. Finally, plants that grow in the water itself, called emergent vegetation, are also critical to the survival of fingerling fish and larval insects that hide among stems and leaves.

Although streams in equilibrium do erode the surrounding landscape and move back and forth over time, they do so slowly, and in balance with their watersheds. As it moves, a balanced stream tends to maintain its shape or sinuosity and its internal structure of pools, riffles, sandbars, undercut banks, and the like. This relative stability provides the platform for the development of complex biological communities both in the stream itself and in the floodplains and other adjacent areas surrounding and affected by the stream. Fine sediments, sands, gravels, and boulders (collectively referred to as bedload), move throughout the system, but are sorted out under normal flows, and in high water, are often redeposited in vegetated floodplains.

If a stream's energy or bedload regime is disturbed for more than a short time, it can fall suddenly out of equilibrium. If stream energy is increased through either water velocity or volume, for instance, the amount of sediment it carries will also increase, through greater erosion. Increases in stream energy occur when its natural sinuosity is disturbed through channelization (straightening out) or when the volume of water is increased, through the removal or change in vegetation, piping of run off water, or addition of impervious surfaces in the watershed. When you straighten a stream, you shorten its length, and therefore increase its slope or steepness. This increases its energy level, and its erosive power. When you increase the volume of water that enters a stream, or the speed with which water enters a stream, you also increase its energy.

Unfortunately, human developments in a stream's watershed often if not usually do both. We clear trees and meadows and install parking lots and roofs, divert rainfall into a system of gutters, pipes, and storm sewers, and straighten, harden, and shorten the streams themselves. The result is a disruption in the stream's ability to maintain equilibrium with the landscape, spelling disaster for biological communities and often manmade structures as well. Increased energy and volume causes much more frequent and severe flooding, and much more rapid erosion of banks as the stream tries to maintain the balance between bedload and energy.

High erosion generally causes the natural channel to deepen (degrade). Instead of allowing flood waters to escape stream banks and spread out into floodplains, degraded streams hold flood waters in these narrower, deeper channels, and multiply the stream's erosive power even further. This causes vegetation to be scoured away, and causes banks to slump into the stream. Overhanging vegetation, undercut banks, emergent plants, and all of the benefits of these features to the stream are erased. If a stream encounters a layer of bedrock and cannot degrade, energy is forced outward, and the channel widens. This effect causes the stream channel in normal or low flow conditions to become stranded from the banks, much shallower, and much warmer, because overhanging shade has been reduced. To illustrate this effect, consider that in developed areas with shallow depth to bedrock, many of the streams that were used by Native Americans as highways for travel by canoe are now much too wide, shallow, and rocky for boating.

If a stream's overall volume and slope are not increased, but the sediment input is, then a somewhat different but equally disastrous imbalance occurs. Sediment input is increased by run off from farm fields, road construction, housing developments, and nearly any activity that allows raw soils to remain uncovered during significant rainfalls. In terms of stream ecology, simple muddy water is perhaps the deadliest of all pollutants. With an increase in sediment the energy of the stream is not sufficient to carry and sort its new, increased bedload. Because the stream does not have the power to move sediments downstream or lift them on to surrounding floodplains, stream bottoms, which would normally be composed of sands, gravels, and cobbles or small boulders, are covered with silt. Aquatic vegetation and aquatic organisms are suffocated, causing disruptions throughout the food chain. Clean, oxygenated gravels and interstitial spaces beneath and between rocks and logs are also filled, suffocating fish eggs and removing critical habitat for stream invertebrates. This is known as an increase in "imbeddedness". As the silt accumulates, the channel becomes much shallower (aggrades), and eventually wider, causing the same damage to vegetative borders as was described above. Aggraded streams also carry higher flooding potential than streams with normal channels.

In the end, it is impossible to separate the health and proper functioning of streams and rivers from their watersheds. The characteristics of a stream are a function of native climate, soils, slopes, and vegetation, but the dynamic equilibrium of a stream is a function of land use and stability in its watershed. We may whish to believe that we can save disturbed streams with "window dressing" programs to plant trees along their banks, or control streams with more and more levees and dikes, but in the end, nature's original design is far superior. Ultimately, the ecologic and physical integrity of the entire landscape, including lakes and oceans, depends on the integrity of streams, and this depends on the integrity of the landscape in which they originally developed. If changes in energy levels and inputs into a stream resulting from disruptions to its watershed remain constant at a new level over long periods of time, the stream will eventually establish a new equilibrium, but this can take decades, and cause serious damage to both natural and manmade systems in the interim, and the new equilibrium is not likely to be as productive as the original.

If inputs are constantly and rapidly in flux, however, the streams do not stabilize at all, and worse yet, slowly lose the capacity to heal. There will always be some life in any stream, no matter how disturbed. We will always have the choice to manage land use and watersheds carefully, or to continue to treat streams as open sewers. The question for us, however, is whether or not these damaged streams will be to our liking and able to provide the resources and amenities we have become accustomed to. As we continue to build dikes and levees, pay for more flood damages, worry about clean water supplies, struggle to find good fishing or swimming, and long for the beautiful and productive streams we once enjoyed, we should ask ourselves whether or not it is worth it in the long run to trade a bounteous, self maintaining natural system for a sterile, dangerous eyesore in need of constant repair.

Other writings by Naturalist William Hudson:

A tribute to the Asters of September,
an essay by Naturalist William Hudson

The Greatest Show on Earth,
an essay by Naturalist William Hudson

Large Scale Stream Dynamics

On taking kids outside,
an essay by Naturalist William Hudson

Another Symbol of the Wilderness in Peril,
an essay by Naturalist William Hudson