Making a pond from a gully

The flow is really not a creek in any way - it is a
tiny but steady surface flow literally about 2 inches wide (max) - then 20
feet down the gully there's no surface water to speak of

Have the water tested for chlorine , chloramine and perhaps florine. There
could be a leaky utility pipe somewhere. Also, do you or your neighbor
have a septic tank?

You may also want to test for disolved solids. If the water has come a
long way it will most likely have more disolved solids than if it is your
local ground water.

I would want to know the water quality before keeping it around. If it is
septic water I would want to divert it back where it came from.