Trees and other large woody material that have fallen from stream banks into the channel provide many physical and biological benefits. Indeed, wood is essential to the health, structure, and biodiversity of streams in our region.
Stream lovers know that wide riparian buffers of native trees and woody plants provide essential benefits. Their shade cools stream edges, their roots absorb storm and flood water and stabilize banks, and their foliage provides food and habitat for birds and insects. But what happens when a tree topples into a stream? Remember, subtle bank erosion and shifts in the course of a channel are natural in wild streams. Streams alter course, banks fail, lightning strikes – a host of natural processes lead to live and dead limbs and whole trees falling into streams. This large wood is incorporated and serves a variety of beneficial purposes.
Downed trees create habitat and diversity. The large tree stranded on a mid-channel bar with its roots buried in the deep channel, or the large limb laying against the bank with its branches in the water, adds diversity to the aquatic habitat. Both continue to shade the water, creating hiding spots for fish and invertebrates. Exposed portions above water provide basking areas for turtles and perches for birds and small mammals. As water flows over and around submerged tree limbs, it scours and undercuts adjacent areas of the channel, creating small pools and pockets and providing additional shelter for aquatic organisms and fish, such as trout. Finer gravels typically collect upstream of large wood as flow slows, creating spawning habitat for some fish species. Indeed, the fallen tree helps feed the aquatic food chain from the bottom up. As it settles in place for the long term, wood provides a surface for algae to grow on and often traps smaller sticks, leaves, and other organic material, food sources for a variety of aquatic macroinvertebrates.
As large wood settles into a channel, it helps maintain stream stability. Depending on the tree species, submerged wood can last for decades. If you walk along the banks of steeper small streams in Adirondack wilderness areas such as Otis Brook in the image above, you'll see wood integrated into established cobble and boulder steps or lining the sides of deeper pools, serving with bedrock and cobble as the building blocks of the channel. Wood embedded across streambanks and into channels creates roughness that can slow higher flows of water and collects sediments, building bed and banks. Large wood also creates connections between the channel and the floodplain – helping to move flood water, sediments, and nutrients out of the channel, to be absorbed and deposited, reducing flood stress and potential damage downstream.
Traditionally, large wood has been removed from streams because it is perceived as unattractive or as a threat during floods. In some cases, judicious removal may be needed for public safety. With only a few exceptions, however, wood should be allowed to move and integrate into stream ecosystems. Healthy, diverse streams have the capacity to store and utilize significant quantities of wood. The enormous benefit wood provides has made it a critical element in natural stream restoration. One example is toe wood – sloped near channel banks, known as benches, built to restore eroded outer bends of streams. On a river the size of the West Branch Ausable River, toe wood is constructed of 60 to 80-foot trees, 18 to 24 inches wide, laid horizontally, angled slightly downward toward the water with 10-15 feet between each intact root crown. Natural materials are layered and anchored on top of the trees so that, in a year's time, the bench will blend in with other healthy banks. Toe wood plays a vital role as water rises. In a bankfull or high spring flood, water will move up onto the bench, slowing its downstream velocity and providing protection for adjacent areas. The toe wood is resilient, easily withstanding high flows and ice, providing habitat, absorbing stream energy, slowing flood water, and angling it back into the channel.
Large wood constructions are essential elements of many Ausable River Association – US Fish and Wildlife Service restoration projects in the Ausable watershed precisely because they are stable in large floods. They also they provide greater opportunities for rapid growth of native plants, creating a rich riparian buffer. Engineered banks constructed only of stone, in an attempt to keep water in the channel, transfer flood effects downstream, and minimize growth of plants and trees. Large wood, such as toe wood in the channel, becomes a platform for new riparian growth on banks. And the cycle continues.
Wood is good.
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