Bags of concrete work fairly well. They set up quickly. But, unless you are careful when stacking them, expect seepage between sacks. Seepage could be significant enough to prevent your new pond from filling. If that happens, use a heavy plastic liner and stack concret on and around the liner. Plus, dig into each bank, and put bags there, too. Creek soils are notoriously porous. When your pond fills, water can go around the concrete dam.

Teach a man to grow fish...
He can teach to catch fish...