Mowing what you can and burning/killing off the woody growths annually is a good solution. That's what I've done for the past 7 years or so on my larger pond for the same reason (too steep in portions) and it's working just fine. Seems like every year after I burn it off the grasses come in heavier...which is the goal...

Dale

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"When tempted to fight fire with fire, remember that the Fire Department usually uses water." - anonymous