There are reams of literature instructing how to perform a dam inspection. If you are interested in this information, you can contact the State Engineer's office for a listing of some of the publications. Root systems for some vegetation can be quite extensive and actually provide a path for seepage. Also, if the plant, tree, etc. dies, the roots will rot, leaving a conduit for seepage. Large trees can be blown over and actually cause a breach in an earthen dam embankment.
Vegetation also provides a home for rodents.
Vegetation can be burned off, sprayed with a herbicide, trimmed etc. If the rodent holes become serious, the rodents must be eradicated and the holes backfilled with suitable material. The outlet works should be exercised from fully closed to fully opened at least once a year to insure operability at high reservoir levels. Access to the outlet works is also important. It may be necessary to access the outlet works at high spillway flows to assist in the draining of the reservoir to prevent overtopping.