I would first try to pull out what ever larger scrub trees you can as suggested above for the reasons stated. However I would then mow the whole thing and let it grow back while you fence the creek. Then graze the new growth.

Grazing tall old mixed fescue and weeds is likely to cause pink eye and they will mostly eat the younger growth anyway. Use an electric fence to subdivide the pasture and intensively graze sections for a few days each and the pasture will gradually come back.

Do a ph test soon and probably needs some lime asap. owner should cover lime cost. jmho.

Jim at Dawn