Results of large woody debris in river/lake In the following questions I am wondering how large woody debris and/or submerged trees enhance river or lake environments.  Also I am wondering how they are detrimental to humans. 

I know that large woody debris and submerged trees are commonly found in rivers and lakes. They may be present by a result of flooding, logging, bank erosion, forest fires, beavers and many other causes.

I am not sure how they would enhance river or lake environments.  Clearly beavers could use them to build dams but how is this good? Is there any other ways they benefit?  

Also, how are they detrimental to humans?  Might they potentially hurt the ecosystem present in the water, which in turn may hurt humans if they are using the ecosystem for resources?
Depending on the fauna within the rivers and lakes, submerged trees can be like reefs in oceans. They provide protection for fish and other fauna and they can also act as nurseries for certain types of fish.

Submerged trees can sometimes be snags in waterways which increases the difficulty in navigating waterways, particularly for larger boats.