Dumb Question Oil Creek

I know this is a dumb question but I do not know the answer and figure this is a good place to get the facts.
I fished Oil Creek this week for the first time and noticed little mounds of gravel scattered throughout the creek what are they from? My guese is they are result of spawning maybe eels?
 
I fly fished caught 8 rainbows on 12/14 adams and was very impressed with bugs fish and scenery. Wish it was a little closer the 1hour plus drive was bit of a drag but worth it.
 
WCW

Thanks for the reply.
I did see a lot of stripped minnows of some kind in the water so maybe you are right.
But like you said it seems strange such big mounds made by minnows, the mounds I saw were  a foot or more in diameter maybe 6 inches high.
Anyone else have any thoughts.............WCW

Maybe Crayfish?

Ok did a real quick search on shiners in PA got this info 1st click...
 
The males defend the spawning territory. Some shiners, including the emerald, bridle, spottail and mimic, broadcast their eggs randomly over gravel, sand or mud bottoms. Several shiners, like the common, rosyface and striped, spawn over the stone-mound nests of creek chubs and hornyhead chubs, allowing their eggs to fall into the crevices of the already built spawning site. Because several shiner species may use the same spawning location at the same time, hybridization is common. Shiners spawn from spring throughout summer, according to their species. They do not defend the nest after spawning.
 
Makes complete sense with  the amount of different minnows I saw that day.
Also a lot of crayfish and tadpoles seen that day I was very impressed with how full of life the creek was......
 
As always thanks for the replies........WCW