Is it a lake or river only? I was told that there was both.

There is the san marcos river and oxbow lake

The river widens to a point where it is called a lake.

We went into the river but not deep enough to swim and the current could be strong

I don't think anyone swims in the river. There is a small lake to swim in the tent camping area.

The San Marcos river flows thru the park and there is a small lake to fish and swim in