[jjallen]

So many lakes do not have rivers flowing in and out. A low spot in an area will become a lake.

Many do have rivers; many don't. Ponds even more so don't have attached rivers.

[bluGill]

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.