Harvey Locke and Richard Hauer: Guest opinion
December 20, 2011
Aspen is a community nourished by gravel-bed river systems and heavily dependent on winter snows for its economy. Aspen, known as an innovative leader and composed of residents seeking the moral high ground, is quite rightly asserting its leadership by taking action to combat climate change.
Climate change is created by increasing concentrations of carbon dioxide in the atmosphere. A significant portion of this is coming from burning of fossil carbon for electric energy in amounts that exceed the earth’s capacity to reabsorb it. The amount of carbon dioxide in the atmosphere is not the problem per se. The problem is the consequent greenhouse effect that changes the climate, which in turn impacts the Earth’s natural systems on which all life depends.
The solution to the problem of climate change is widely agreed to be two-dimensional: reduction of emissions of carbon into the atmosphere (called mitigation) and adjusting to climate change (called adaptation). Climate change is already under way and causing changes to Earth’s natural systems that are getting more acute. It is urgent that we address this two-dimensional problem.
Aspen needs energy to maintain its economy and quality of life. People concerned about climate change have become interested in energy sources that are less carbon intense. This in turn has created renewed interest in hydroelectricity, which is often described as renewable or “green energy” because of its low carbon intensity. There is the additional desire to have locally produced and distributed energy systems to promote efficiency and awareness of where energy comes from, which in turn has created interest in a “micro-hydro” project that involves Castle and Maroon creeks. But is this the best way to use these creeks to address the two-dimensional problem of climate change?
With hydroelectricity, there is a benefit on the mitigation side, but it creates a problem on the adaptation side of the climate-change equation. Dams and diversions profoundly damage the adaptive capacity of the Earth. A solution on the mitigation side that has a material negative impact on the adaptive capacity of the Earth is not really a solution to the integrated problem of climate change.
A gravel-bed river consists of water that flows through cobbles that vary in size from footballs to golf balls. These gravels form the bed and banks of the river, extend across the floodplain until they hit bedrock and are in dynamic balance as floods move them and water flows through them. In such systems, the water you see flowing in the river itself is just “the tip of the iceberg.”
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The riparian area where the trees grow on the gravels of the floodplain is underlain by river water that flows beneath the surface. So connected is the floodplain to the river that water pumped from a well many yards back from the river channel can actually draw down the flows in that river. If you pour tracer dye down such a well, it will appear in the river, often within only a few minutes, many yards downstream.
Similarly, water removed from a stream by diversion or consumption will reduce the amount that flows through the gravels of the riparian zone. The riverbed is not like an impermeable sluice box through which the water flows in isolation – rather it is more like a sponge with a channel cut halfway into it where the entire sponge is wet. Reducing the flow dries out the sponge. This interconnection between the water and the gravels is what characterizes a gravel-bed river system.
Spring floods bring larger volumes of water with higher energy that rolls the gravel over, creating spaces between the cobbles in the riverbed and on the floodplain. In these gaps between the stones, trout lay their eggs (redds) and the insects that trout eat spend most of their life cycles. The sediments that these floods deposit create soil where cottonwood seeds can grow. Indeed, over time the entire river system moves around the floodplain in a shifting pattern.
With spring floods, the river may leave its current gravel channel and cut another one in the floodplain. The old channel becomes floodplain that is colonized by seedling trees. These peak flows that drive the system are thus essential to the renewal of the life of the river system from the fish to the forests. The water flows in and out of the gravels many times per mile of channel. River water flowing out through the floodplain waters the springs and ponds of the riparian forest, sustaining fish nurseries, frogs and birds. Spring floods are therefore essential flows, not surplus flows.
The water in the river channel, riverbed and riparian zone are important at the scale of the entire landscape. The various parts of the river corridor, composed of channel waters, river water flowing through the gravels and the floodplain surface, interact to create habitats that are used by as many as 85 percent of all the species in the landscape at some point during the year. The river corridor functions as the ecological backbone of the entire landscape.
The removal of water from a gravel-bed river system affects much more than the amount of water available for things that live in the river. Eliminating floods ultimately eliminates the space for new growth; the forest simply ages and dies without being replaced. The gravels eventually tighten up, leaving no room for the fish to spawn or for insects and other aquatic life. The floodplain becomes simplified, the cottonwood forest is lost, birds that use trees disappear, and the vitality of the region is diminished.
With climate change we know that species are confronted by three possible scenarios: adapt, move or die. Very few species will be capable of adapting. The species that can move will move north or move upslope to find the lower temperatures they are adapted to. They will do this by following corridors. Gravel-bed river systems that go upslope in mountain systems are the natural corridors that will benefit the most species and thus are essential to keep ecologically intact in our effort to adapt to climate change.
The widely used models that calculate instream-flow needs do not address the interaction of the river with the floodplain. They do not help us assess the long-term impact of dewatering a gravel-bed river. They do not take into account the need for the power of the river flood to mobilize gravel nor the function of subsurface flow or riparian forest renewal. This does not mean that removing any water from a gravel-bed river is a disaster. But it does mean we need to be very cautious, remove as little water from them as possible and maintain processes like flooding and connectivity between the channel and the floodplain. Put another way, the benefit of diversion and dewatering of creeks on the mitigation side ought to be very large before such an action is undertaken as the costs on the adaptation side are very high.
In a world under stress from climate change, we believe that usually the best thing to do with gravel-bed rivers, like those in Aspen, is to keep them intact so they can serve as vital corridors for adaptation, which is essential to the long-term ecological viability of the region.
After all, we are only concerned about climate change because of what it will do to Earth’s natural systems on which we and all other forms of life are wholly dependent.
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