Snakes Catching Fishes

I spent yesterday morning observing dragonflies on the Parmer Village end of Lake Creek Trail. Starting at the Parmer Village pond I eventually worked my way to the last dam on the creek. With recent rains, the creek was high enough that water was spilling over the dam. Our creek is loaded with water snakes in the Nerodia genus, mostly Blotched Water Snakes. (These are all non-venomous. I have yet to find a venomous snake anywhere in our neighborhood.) As I scanned the shallow water pouring over the dam, I saw this Blotched Water Snake maneuvering around the base of the dam, right where the slope stops:

It came to a rest with only its head extending above the bottom of the slope:

And about 5 feet away I saw two more of these snakes doing the same thing, hidden under some flotsam:

What were these snakes up to? I decided to sit down and watch them for awhile to find out. I guessed that they were waiting for small fish to get washed over the dam, and they'd catch the fish on its way down. I was almost right, but the truth was surprising.

Every now and then a small fish would emerge from below the dam, and like a mini-salmon returning to its spawning grounds, it would attempt to swim up the slope! These were the fish the snakes were after. The water wasn't even deep enough for the little minnows to be completely submerged, but they were fighting their way up! I never saw a snake catch one and I never saw a fish make it all the way up the dam.

Why were these fishes making this herculean attempt? Do they know that the water below the dam will dry up, and their only hope for long-term survival is to make it into the permanent pool above?

Here are some additional photos from the morning, mostly of dragonflies. It was a fun morning!