Resilience–the ability of a substance to spring back into shape—is a much-desired characteristic. Resilience is also the capacity to recover quickly from difficulties, or toughness. As people and societies, we crave resilience to the difficulties imposed upon us, including those of our own making. Accelerating change in the climate that influences every aspect of our existence is one of the most frightening of those difficulties. Increasingly, I see news headlines and scholarly articles discussing how to increase resilience to climate change.
In my part of the world, the drylands of the interior western United States, declining precipitation is the most frightening aspect of climate change. Limited water has long defined this region, as recognized in the evolutionary adaptations of plants and animals, the cultural adaptations of Native Americans, and the technological adaptations of European Americans. We have dammed, diverted, and pumped our surface and ground waters into a highly engineered plumbing system to serve our deepening thirst, and we have sometimes saved a pittance of water to aid the survival of native plants and animals. But the continued functioning of our elaborate water engineering depends on continued rain and snow. It is these that climate change jeopardizes, so we speak of developing resilience in our water supplies.
The Rocky Mountains are Colorado’s water tower. Having largely polluted or depleted our shallow ground water supplies, we depend heavily on snowmelt that spreads outward in all directions from the Rockies. As a river scientist, the resilience of these rivers is often on my mind and governs the questions that I ask.
River scientists have thought about this for decades and we have a good understanding of what has been done and what should be done to enhance river resilience.
What creates resilience in rivers? Buffers.
Rivers create buffers of space and connectivity. Rivers are channels connected to floodplains so that water and sediment rushing downstream during a flood can spread across the valley bottom and move downstream more slowly, leaving sediment as nourishing floodplain deposits in which new plants can germinate. A channel connected vertically to a thick wedge of underlying sediment can remove harmful nitrates and increase the dissolved oxygen of river water that filters through the sediment before returning to the surface. A channel can also allow fish and other organisms to move up- and downstream to find refuge during flood or drought.
Rivers also create buffers of diversity. Rivers promote physical diversity by creating side eddies, pools, side channels, differently sized sediment, and floodplains with lakes and marshes. Physical diversity means habitat diversity and habitat diversity supports biological diversity. Protected from predators, juvenile fish can grow large in the shallow, warm waters of side channels. Fish that need muddy bottoms and cattails can thrive in the floodplain marshes, and fish that need clear, cold water and gravel beds for spawning can thrive in the main channel.
Plants that root below the water surface can grow in new ponds and plants that like wet soil can grow in older ponds that are gradually filling with sediment. Spatially diverse channels and floodplains provide sites of refuge during floods, droughts, and other disturbances – some portion of the river valley is more likely to have slower flow during the flood or to retain standing water during the drought.
Space, connectivity, and diversity are the product of river work through time. The environment through which the river flows is neither passive nor uniform. Trees fall into the channel, forming logjams that obstruct the flow. Big boulders resist erosion, forcing the channel to bend around them. During floods, water ponds at the logjams and boulders, spilling onto the floodplains and surging into the subsurface to create lateral and vertical connectivity.
Natural rivers have a high level of resilience: a plethora of plant and animal species have persisted in rivers for millions of years, despite flood and drought. I worry about their continued persistence under a changing climate because of how people have reduced the resilience of rivers. We have constrained their space with levees, channelization, and stabilized banks and beds. We have disconnected them with dams, diversions, and levees. We have homogenized them by regulating the flow, removing obstacles within the channel, and manipulating the channel boundaries. To the extent possible, we have made rivers into canals.
There is no quick fix for the centuries of engineering that have reduced the resilience of rivers and made river ecosystems and human societies more vulnerable to a warmer, drier climate. But there are remedies, and these must start with recognition of the buffers that create and sustain river resilience.
Featured Image Credit: Wheaton BM fire by Joe Wheaton. Used with permission.