Hydrology 1: Why Rivers Need Floods
“That salmon is us. All of us. Whatever happens to that salmon is going to happen to us. If we can’t protect the salmon and its habitat, we can’t protect ourselves from the same things that are driving the salmon toward extinction.”
Floods. Every year, they make the news, wreaking havoc with livelihoods, and sometimes the very lives of people living along rivers. The same story, often with the same players and in the same places, has been told since time immemorial. Most of the time, the river is peaceful, inviting us to its beauty, inviting us to play, or to work or live, on its banks. But those banks, and even the floodplain, are actually a part of the river itself, which it reclaims during floods.
When a flood affects us, we often act as if it is something new, something unexpected. We look only at the destruction, and not at the role that floods have always played in the long-term health and stability of the river. Rivers owe their very form, and many important parts of their habitat, to floods. And it makes sense that fish which thrive in this habitat have evolved to make use of, and depend on, the occurrence of floods. Indeed, the recovery of fish populations that rely on rivers, like the seven species of Pacific Northwest salmon, depend on floods.
Expecting the next flood
Scientists and engineers have developed a language for talking about floods based on their probability of occurrence in a given year. This presupposes that we know the river will flood. It’s just a question of how likely it is to reach a certain size. For example, a big flood might have a chance of one in 100 (or, 0.01) of occurring this year. A moderate flood might be one that has a chance of 1 in 2 (0.50, or 50%) of occurring this year; and a smaller, but still moderate flood would be one that has a chance of occurring twice in three years (2/3, or 0.66). Another way of talking about these same three floods is to label them by the average time, in years, between occurrences of a flood that big or larger. To do this, just take the inverse of the probability of occurrence. So, the flood with probability 0.01 becomes the “hundred year flood,” and the floods with probability 0.50 and 0.66 become the two-year flood, and the 1.5-year flood, respectively.
Moderate floods vs. large floods: What floods accomplish
Floods have a lot of power to move things around. They move sediment (gravel and sand), and logs or trees that have fallen into the river. Floods also erode riverbanks, adding more sediment to the moving water and causing trees to fall in. Large floods have more power to move things around then moderate floods. But bigger floods happen much less frequently. So, it turns out, that over the long-term average, it is those moderate floods, and not the big ones, that move most of the sediment, that do most of the work transforming the river, because they happen so much more often. And, because of that, it is these moderate floods that determine the form (width, depth, shape) of the river. The moderate floods determine where the floodplain begins, while the big floods determine where it ends.
The alluvial river develops, over many years, a shape which is just the right width and just the right depth to move all of the sediment during a moderate flood. A flood that just fills the river channel up to the level of its floodplain is called a bankfull flow. And usually, but not always, this bankfull flow is about equal to the 1.5-year flood.
As the flow becomes greater then bankfull, it spills out onto the floodplain. As it does this, it carries with it sediment (sand, and silt) in suspension. This sediment settles out as it flows in a thin sheet across the floodplain. This is what, over the centuries, builds up and renews the floodplain soil.
Because the water is spilling out onto the floodplain once it begins to rise above bankfull, the force of the water on the streambed and banks is relieved. That is, the velocity of water and the force it exerts on the streambed and the banks is smaller when water can spill out over a floodplain than it would be if the water continued to build up depth in the channel and not flow outward. This is part of the balancing of forces that allows the shape of the channel, determined by bankfull-flow-sized floods, to transport all of the sediment in motion in the river, and to determine the size of the streambed substrate (sand, gravel, cobble) as well. If it were the big floods determining the streambed substrate, there would be no gravel or sand bedded rivers. Sand and gravel would all be washed away. Likewise, if the big floods determined the shape of the channel, the channels would be very wide, and consequently they would not be able to move all of their sediment during most of the time, since a wide channel would be shallow and shallow water doesn’t have much force to move sediment.
Moderate floods keep vegetation from encroaching into the channel. They also renew the streambed, loosening the gravel, flushing away waste products, pollution, and fine sediment that might otherwise build up. They scour away silt and fine sand that clogs the pores between gravel. In this way, when fish lay eggs in the gravel, water can flow through, aerating the eggs. In those parts of the river close to saltwater, moderate floods help to maintain suitable salinity in the important areas called estuaries, where freshwater and saltwater mix.
Large floods determine the extent of the floodplain. This periodic standing or flowing water on the floodplain has a profound impact on the soils in the vegetation. Different plants, particularly trees, have different tolerances to this frequent or infrequent inundation. Some trees, such as the black cottonwood found in the West, depend on large floods to reproduce and become established. Seedlings will sprout on their sandbars and riverbanks, and grow at a rate where their roots can follow the receding water in the soil, becoming established before the hot dry conditions of summer would otherwise kill them.
Big floods, because of their powerful flow, can cause the river to change course abruptly. Water moving across the floodplain can become concentrated in low spots, forming a new channel which erodes its way back upstream to connect with the existing channel. This forms a secondary channel or “side channel,” and if the flow is strong enough, the side channel can become the new main channel, leaving the old main channel with less water. Scientists call this event an “avulsion,” because it can suddenly cut off a portion of the former channel. The river has moved, or “migrated.”
This new channel becomes fresh new habitat. Side channels are an important component of fish habitat. Fish use them to escape from high velocity water during floods, and some species of fish, such as the Chum salmon, use them exclusively for spawning. Once a side channel has formed, if it doesn’t become the main channel, it gradually begins to fill in with logs, sediment, silt, vegetation. So for side channels to exist, there has to be ongoing channel migration, creating new side channels as old ones dry up.
Side channels can also form during moderate floods. As the water goes around the bend in a meander, the high velocity water presses outward against the soil making up the bend, causing it to erode. So gradually, the meander expands and widens. Eventually it gets so wide that it slows down the flow, causing the river to find itself a shortcut across the neck of the meander, cutting it off. The former meander loop then becomes a blind channel, another type of side channel. This entrance to this blind channel eventually becomes filled in, separating the channel from the river to form a floodplain lake, called an oxbow lake.
In rivers that have trees growing near them, which, really, is most of the rivers in the world, big floods move these trees around, uprooting them, carrying them, and depositing them in accumulations called logjams. Logjams also can abruptly alter the course of a river. A logjam can form, diverting the river into a side channel or onto its floodplain, forming a new side channel. And this can happen rapidly, within a single flood. Logjams themselves are a very key element in fish habitat, providing a variety of different water velocities, streambed zones of different sediment sizes, a labyrinth of crevices to provide cover, protecting small fish from predators and forming a substrate for the insects that fish eat.
In areas with dry climates, large floods saturate the floodplain soils in a way that would not otherwise occur. These soils can then remain moist long into the dry season, as this stored water slowly percolates back into the river channel through the soil, helping maintain the perennial flow. Without this flood saturation, the river would dry up during the summer
Fish and other aquatic life, including the insects that make up the diet of many fish, depend on floods for carrying out their lifecycle. The flood can provide a trigger for fish to migrate upriver from the ocean, as well as providing the necessary water conditions for them to do so. Floods can trigger insects to enter a new phase in their life cycle. During floods, the floodplain becomes a place for juvenile fish to escape high velocity water in the channel, creating a nursery area. Some species of fish even spawn on floodplains during floods.
Floods deposit nutrients on the floodplain. In places like western North America, the carcasses of spawned out salmon, deposited on the floodplain, were historically an important source of nutrients for the forest and animals that lived in it. Even with the greatly reduced salmon populations that exist today, scientists can track nutrients that originate in the ocean in trees and in animals such as bears and birds such as dippers, finding that these nutrients derived from salmon make up a large proportion of the available nutrients in the ecosystem.
In tropical parts of the world, such as the Amazon, fish depend on swimming out into the floodplain to forage for plant or insect food, during the season of floods. Floods in many parts of the world distribute the seeds and fruits of plants growing on the floodplain.
Finally, big floods are a time of renewal. They can reset the biological clock of a forest back to zero, creating bare patches of new soil (freshly deposited), and the moisture conditions necessary for the cycle of forest growth to begin. Shrubs and young trees become established, giving way over years to larger trees and the vegetation making up the mature forest. Without this periodic resetting of the clock, the younger phases of the forest will disappear from the floodplain, along with the animals and plants that live in those younger phases. Scientists call this process forest succession. Floods are needed to maintain the diversity of forest types and ages on the floodplain. Without floods, the mature forest would also disappear since the trees got there by reproducing under earlier conditions, from an earlier stage in the succession process. Eventually, these trees would be replaced by something else.
Living with floods
When people live on the floodplain or along the river banks, not realizing that these areas are actually part of the river, it is no surprise that they become victims of floods. In modern times, this has led to the perception that floods are “bad” and that they need to be eliminated through massive, expensive engineering projects. Dams alter the flow patterns in the river, eliminating the large floods, changing large floods into longer-duration moderate flows or simply storing the water to release it at another time of year. Dikes and levees prevent the river from spreading out onto its floodplain, forcing it to remain concentrated in the main channel.
Scientists have been aware for a long time that dams and levees cause problems for the fish that depend on the river, with its cycle of floods and calm periods. Initially, however, they were mainly concerned about having enough water during those calm periods, particularly during the summer, for the fish to live. Operation of dams was regulated based entirely on trying to get minimum flow requirements during times of the year when water is low, and providing for artificial fish passage around the dam. It wasn’t until the 1970’s and 1980’s that river scientists began to argue that rivers require a much fuller spectrum of flow conditions to sustain their ecological health, and to maintain the habitat needed by native fish species. This fuller spectrum of conditions needs to include floods, not just to provide occasional high water conditions, but to reestablish the functions that floods perform on the streambed, floodplain, and habitat.
This led to the idea that, if we must have dams, we should still allow certain floods to happen, at their full magnitude. For example, the first flood of the wet season would be good to fully retain, since this is a trigger for fish migration, providing conditions needed for that journey. This first flood often does much of the work washing out accumulated debris and toxins. A few of the larger floods could also be retained, or even artificially produced, to help maintain the condition of the streambed, riverbanks and floodplains.
This newer approach to living with floods would be a way to consciously select how much of the original ecological condition can be retained, rather than writing off the many once-productive dammed rivers as lost causes.
Without a reconciliation to the necessity of floods; without the processes that those floods perform creating and maintaining habitat, dammed rivers will evolve into something quite different from what they once were, and something quite alien to the needs of the fish, like salmon, that evolved with them. Those salmon will then disappear, which is a tremendous loss to us.
For a detailed discussion of these ideas and others for more enlightened management of Rivers and the fish that depend on them, read Sandra Postel and Brian Richter’s (2003) classic, “Rivers for Life.”
Bibliography:
Postel, Sandra and Brian Richter, 2003. Rivers for Life. Island Press, Washington, D.C.