A tree that a beaver chewed on and dropped will look something like this:

[Linked Image from freerangestock.com]

Only you can determine if they are good or bad for the pond. Unfortunately you cannot tell them NOT to tunnel in your dam, or NOT to cut down a certain tree.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).