Reservoir Help?

The property we are moving to has a natural seasonal reservoir (during spring run off) I know almost nothing about how to contain ground water effectively (other than a pool or other container)

I have posted pictures on my BLOG If any one has some ideas for me on how we might be able to improve this so water last more than just during run off that would be wonderful! The current owner says in dry years (like we have experienced the last few years) it doesn't even hold water through planting season. In wet years it has held water through June.

(cross posted at Permies Ponds )

No ideas?