By David Cronenwett
The waterways of my home landscape have begun to swell with an abundance of mountain snowpack, loosened and released by warmer days. The Teton River, its vegetation barely leafing out, spills from a mountain canyon and barrels across the prairie, coursing powerfully toward the distant sea. For me, rapidly moving water is always an impressive display of energy. But while the eye is drawn to the river in its seasonal flood, habitat created by it— the riparian zone of willows, cottonwoods, birches and other water-dependent species—is dwarfed by the larger grassland it crosses.
“Riparian” derives from the Latin Ripa, or river bank, and refers to the unique biome surrounding rivers and streams, though similar vegetation can exist around some lakes as well. The riparian zone comprises only 3 percent of Montana’s land base, yet it is in this lush, thread-width habitat that nearly half of all our wildlife species spend part of their life cycles. From grizzly bears to beavers and orange-crowned warblers, the riparian world is one rich in species diversity, water and light. Each year, untold numbers of migratory birds rely on Montana’s riparian areas for food and shelter.
The presence of a river’s water (and seasonal flooding disturbance), create ideal conditions for a plethora of trees and shrubs to take hold, most notably, the cottonwood (Populus spp.), of which Montana has four common types. Cottonwood trees can become stately, large-diameter specimens along major rivers, forming extensive riparian forests in some locations. The contributions of this species to the health of riverine habitat can’t be overstated. Among other functions, cottonwoods provide critical nesting sites for many birds, play a major role in the life cycle of aquatic insects, and positively influence the morphology of streams by ultimately falling into them, thereby creating eddies and structural diversity in the stream channel. Shade cast by trees has a cooling effect on a river, important for cold-water species like trout and other native fishes. This tree, however, is very picky when it comes to reproduction, requiring extremely fine, gravelly soils to germinate. Where annual flooding no longer takes place (such as on human-dammed waterways), the flood-adapted cottonwood does not fare well.
Riparian areas are by their nature, ecotones; habitats that overlap and blend with nearby environments. Ecotones tend to be very diverse in species. In open landscapes especially, rivers attract wildlife seeking shelter and in many cases, use them as travel corridors. The startling movement of grizzly bears from the Rocky Mountain Front to eastward prairies in recent years is due in part to the vegetative cover that rivers and streams provide. This same vegetation stabilizes stream banks and absorbs the tremendous power of floods as well as retaining and regulating water flow over time. These are invaluable “ecosystem services” that directly affect human quality of life for those living in the floodplain and well beyond.
People too, have a strong predilection for riparian habitat from both an aesthetic and perhaps even evolutionary standpoint. Hunters have long known that many wildlife species are powerfully drawn to streams and avid birders head there for the same reason. Wood warblers, flycatchers, vireos, woodpeckers, raptors and others can be reliably observed along Montana’s waterways. Alas, these landscapes of willow and water are easily susceptible to damage from a variety of causes. Invasive plants, residential development, improper grazing and the damming and dewatering of rivers can impose great ecological impacts. Given how rare healthy riparian habitats are, yet how much they contribute to wildlife populations, we should manage them wisely and make restoration efforts where feasible.
During spring, rivers and streams are places where nature’s elemental forces of change are easily felt. When water recedes and days shorten in autumn, the painted golds of our riparian woods are one of the greatest pleasures to behold in that season’s low-angle light. Montana’s riparian zones are special places worthy of our greatest affection but to survive into the future, these habitats will also require our greatest care.