Although they may not commonly be viewed as hotspots for biodiversity, gravel-bed river floodplains are by far the most important feature for nature across the landscapes of western North America.
This is because gravel-bed rivers disproportionately create high diversity of habitats, concentrate nutrients for growth, and provide corridors to link populations of species that would otherwise become isolated.
More than just river channels carrying water off the landscape and channels used by fish, gravel-bed rivers are essential to the life requirements of a wide variety of aquatic, avian and terrestrial species from microbes to grizzly bears.
What’s more, in a recently published article authored by myself and interdisciplinary team of colleagues, we found that gravel-bed river floodplains will be critically important for native species under the increasing pressures of climate change now and in the future.
Home to many creatures
Unlike meandering sand- and silt-bottom rivers like the Missouri and Mississippi, gravel-bed rivers flow over deep beds of gravel and cobble that form floodplains which function somewhat like a sponge.
These types of rivers and floodplains form a network of complex habitats and corridors of connectivity across the mountain west of the United States and Canada, a landscape once covered by glaciers. In our shared mountain region from Yellowstone to Yukon, the complex habitats of gravel-bed rivers are maintained through time by flooding, channel and gravel movement, and new life.
Although occupying less than 3 percent of the area, gravel-bed river floodplains contain over half of the region’s plant diversity. More than 70 percent of the region’s bird species use these river floodplains for some critical stage of their life. The large, iconic deer and elk of the region use them year-round, but most extensively in winter as these animals look for food and habitat.
Wolves not only follow the elk or caribou around on the floodplains, but den almost exclusively on river floodplains. Gravel-bed rivers are also critically important sites for grizzly bears, particularly early in the spring as bears emerge from hibernation and are looking for early, tender shoots of vegetation.
These gravel-bed rivers have large volumes of water that penetrate the cobble and gravel of the floodplain, only to return to the river channel kilometers downstream. In the gravel-bed rivers themselves, trout and salmon feed on aquatic insects that live on the river channel bottom. Fish also eat the insects that live hundreds of meters out from the river channels in the gravels of the floodplain and that migrate back to the channel to emerge as adults.
The exchange of water, nutrients, and insects between the floodplain and the river channel feeds the river and cools the channel in the summer and keeps it warmer in the winter. Many of the trout and salmon of our western rivers are dependent on this water, temperature, and food exchange between the river channel and the floodplain for proper spawning sites. Moose, elk and bears all need the spring grasses and herbs that come early on the floodplain because of the surface water and groundwater exchange.
Despite their ecological productivity, however, floodplains are among the most endangered landform types worldwide.
They are flat, rich and attractive areas with abundant water for municipalities, agriculture and recreation. In most mountainous systems, they are the first to be converted to permanent human settlement, agriculture, industry and developed for transportation.
While there are many protected areas in the northern Rocky Mountains of the United States and Canada – Yellowstone and Banff National Parks are two examples – humans have altered the structure and function of the gravel-bed river floodplains both outside and inside these protected areas.
Many of the region’s cities such as Calgary, Missoula and Kamloops in British Columbia, were pioneered along the edge of river floodplains when these rivers were important for commerce. Virtually every city near a river has deliberately encroached onto the neighboring floodplain and subsequently built levees and hardened structures, such as rip-rap to prevent flooding and damage to infrastructure. Unfortunately, these prove to be inadequate when very large, but highly repeatable floods occur.
As the effects of a rapidly changing climate take hold, gravel-bed river floodplains will play a vital role in sustaining both nature and culture. Indeed, climate change will further stress habitats and populations that already have been impacted during a century of development, which threatens the sustainability of the entire region’s biodiversity.
For example, trout and salmon are especially vulnerable to climate change because their survival is dependent, from eggs to juveniles to adults, on an abundance of clear, cold, connected and complex habitats that are concentrated on gravel-bed river floodplains. Likewise, birds, deer and elk and large predators are dependent on the complex mosaic of habitats that are impacted by dams, municipalities and housing developments.
Gravel-bed river valleys, when changed by human populations and infrastructure, fragment these wildlife populations. But these changes can be reversed or mitigated by managing floodplains to better resemble their natural state of flooding and channel movement around on the floodplain.
Change in conservation practices
Throughout North America the ecological restoration of streams and rivers has primarily focused on increasing habitat heterogeneity, or complexity, in a static fashion. The most common practice in stream restoration has been reconfiguring channels and adding boulders, large wood structures and channel-spanning weirs to enhance habitat and restore biodiversity.
However, these approaches have been shown to be largely unsuccessful because they often lack restoration of the natural dynamics of rivers interacting with their riparian zones.
Rather, successful river restoration and renaturalization has been achieved along tens and even hundreds of kilometers of gravel-bed river by reintroducing naturalized flooding regimes. These restore the dynamics of gravel and cobble movement of the channels and create new habitat for plant succession and a diversity of animals.
Many of the great rivers of the world originate in mountainous regions where gravel-bed rivers and floodplains play an essential role in the biodiversity of life in that region and in the quality of human well-being. An overriding question remains: How do we resolve the enormous gap between what scientists know is needed to maintain and restore functioning floodplain and gravel-bed river systems, and the neglect by land-use managers, energy-planners and society as a whole?
Implementing conservation policies that reflect this scientific understanding will require a paradigm shift from conservationists and river managers alike to prioritize maintaining natural dynamic rivers where they exist or restoring them where ever compromised.
Regional biodiversity in the interior mountains of western North America will depend on the the natural processes of gravel-bed rivers and their floodplains to sustain our fisheries, birds, deer and elk, and our carnivores.