Epic fail, I couldn't catch a single minnow in a pool full of minnows!
I placed a trap baited with dog food and bread in my deepest pool for five hours. I only managed to catch 2 GSF.
I placed the trap near the edge of the strong flow because that is where I observed the largest school of minnows.
I now think the current was too strong there, and I only saw the minnows there because that is where they initially sheltered after I startled them walking up to the edge of the bank.
Thanks to all for the good advice, I think the problem was "operator error".
Fortunately, I will have many more chances in the future to try sampling the creek. The good news is that I believe I observed three different types of minnows. (Unless some were fish fry. My eyes are certainly not good enough to distinguish between fry and minnows without a lot more calibration.)
