Lake Travis might be better characterized as a river, but if we call it a lake that still has nothing to do with the objection in your parent comment. Lake Travis's surface area is 19,000 acres. A 12'-diameter pool cannot be called a "lake" because it is far, far too small to qualify. Inflow and outflow don't enter into it.

Note that by the proposed definition, all rivers and all segments of any river are lakes; outflow is always equal to inflow for any persistent body of water.

Personally, I'd be more comfortable choosing whether to call Lake Travis a "river" or a "lake" based on whether there's a current on the surface. If yes, it's a river. If not, why not call it a lake.