UVM professor Jeffery Huges and Susan Spackman published a paper in Biological Conservation in 1995 talking about the importance of corridor width along streams. They did the study on third and fourth order streams here in Vermont. 

They found that:

-Biodiversity in riparian zones is exceptionally high compared to terrestrial or aquatic sites alone. 

• In other words there is no standard corridor width that corresponds to stream size. 
• Larger river like the Winooski may need very large corridors to support maximum biological richness. 
• Because land-water interfaces become more complex as stream order increases, it follows that corridor width will increase but Huges and Spackman advise assessing this on a site by site basis. 

Basically riparian corridors are unique ecological areas. Silver maple flood plains are indicative of this and floodplain systems in general are often structurally and biological complex areas. They increase aquatic species richness as well as terrestrial richness. 

http://www.deepdyve.com/lp/elsevier/assessment-of-minimum-stream-corridor-width-for-biological-dqmXmFnuMs?key=elsevier