Though both arguments—tree roots help maintain the integrity of the levee, and tree roots help degrade the integrity—have merit, having nothing but grass on the levees does ensure that future levee improvements will be stronger, as the natural dying-off of trees can cause the decaying roots to weaken the levee regardless of the possible levee strengthening healthy trees provide.
And in this case, we would have to agree with the engineers regarding flood safety, better safe than sorry.
An excerpt from the Sacramento Bee article.
“The federal government is pressing forward with a policy that could require trees to be stripped from California levees, eliminating what shade and wildlife habitat remain along the state's rivers.
“An interim agreement appears likely to shield the state's levee habitat until 2012. But after that the U.S. Army Corps of Engineers could impose the new rules, which would allow nothing but short grass on most Central Valley levees.
“Levee maintenance agencies can seek an exemption – called a variance – but even then, many trees would not be spared.”