Many ponds have seasonal streams. In spring (when the snow melts) they get connected to the local river system by a shallow stream, then when the water levels

Many ponds have seasonal streams. In spring (when the snow melts) they get connected to the local river system by a shallow stream, then when the water levels go down the stream disappears but the pond remains. Also beavers may flood a large area, then move on leaving a formerly connected pond isolated again.