Complex sentence: Water enters the lake from Outlet Creek which is connected to Lake Hindmarsh to the south, when full the lake is controlled by the sill level of Outlet Creek, which once breached drains the lake to the north.
Split into simpler sentences: Water enters the lake from Outlet Creek which is connected to Lake Hindmarsh to the south. When full the lake is controlled by the sill level of Outlet Creek, which once breached drains the lake to the north.