Large woody debris (LWD) is the technical term used by environmental scientists to refer to any fallen tree, log, stump, root wad, or pile of branches that form part of the local ecology of a stream, river, lake, or et cetera.
LWD placement is often an important part of forest management and habitat restoration projects. Deforestation, channelization, well-intentioned but misguided 'river clean-up' programs, or mass removal of large trees in decades past can decrease the amounts of LWD in riparian settings. This is important for two major reasons:
Stabilizing Shoreline: Large logs, trunks, and root wads prevent erosion of banks and shorelines both by providing a barrier along the banks and by causing the current to slow and meander. In a healthy woodland, large trees grow and fall regularly over the course of decades. Shoreline development, deforestation, or a change in local flora can change this cycle. As older LWD rots or is washed away in storms and floods they will not be replaced, leading to increased erosion. Banks will be washed away or undercut by fast moving water, and will eventually collapse.
Root wads are frequently used to help stabilize banks, being durable, compact, and useless for any other purpose. In many areas helicopters are used to fly them in and place them. Logs are also useful; 'logs' in this case may refer to entire tree trunks, allowing for long stretches of a riverbank to be protected with comparative ease. Root wads have a secondary benefit of providing more protective habitat than logs.
Habitat Loss: A number of species, from algae to insects to snakes to fish, live in the sheltered crannies created by LWD. For example, Salmon breed in stagnant pools created by large blockages, and their young are protected by hiding under logs and branches. They also feed on insects that thrive in protected crannies and shallow, slow moving pools created by LWD, and that lay eggs on underwater branches and logs.
LWD influences channel morphology, causing streams to slow and meander, resulting in a slower current, increased sediment deposits, and wider and more shallow channels which are warmed more quickly by the sun. This creates better conditions for fish to hunt and migrate. In turn, these conditions (including the increased fish populations) increase the food supply for predators. The LWD also provides perches for birds and reptiles.
Whole trees, with branches still attached, are often preferred for creating new habitats. The small branches toward the apex of the tree are particularly effective at slowing the current and providing protection for small fry.
To keep these evils from plaguing our streams and rivers, park rangers and other concerned individuals will haul in various forms of LWD and leave them strategically draped along our waterways. Of course, the whole point of LWD is that they occur naturally, if we leave nature to its own devices. Naturally we have any number of common words for LWDs that we used before the biologists got involved. These terms include driftwood, which can perform many of the functions described above in its coastal habitats; logjams and beaver dams, which are a specific form of LWD in which the waterway is completely blocked, forming a natural dam; and of course, the everyday words 'log', 'fallen tree' and 'deadwood'.
LWD is a term used specifically for water-based timber. If a tree falls in the forest, it is known as coarse woody debris.
References:
Streambank Habitat Enhancement with Large Woody Debris by the US Army Corps of Engineers Ecosystem Management and Restoration Research Program (.pdf)
Large Woody Debris Fact Sheet by the Connecticut Department of Environmental Protection (.pdf)
Large woody debris by Heidi Hopkins.
Large Woody Debris as an Ecological Function from the King County, Washington website (on the Puget Sound).
Wikipedia: LWD