It is important to keep them away from more than just the dam -- although that is by far the most critical. They can weaken the dam. Worst of all, if they should fall over during a saturated period, they can take out your dam.
I tried finding some articles from the late-80s and mid-90's regarding what appeared to be a very mysterious water loss in the Potomac River area that provides of the potable water to Washington DC, and the adjoining Maryland and Virginia suburbs.
During a serious drought in this area a number of years ago, the lower fresh-water parts of the Potomac river water levels were dropping far more than expected or predicted. They eventually found that large trees on the river banks were each respirating many hundreds of gallons of river water each day through their roots to and out of their leaves. When expanded over more than a 50 mile river stretch, and many tens of thousands of trees, the water loss to respiration was enormous.
So, just a thought from my addled brain ...
Ken
