In a river, large rocks are transported by

slumping
solution
saltation
traction

traction

Traction is a geological process in which a mechanical force transports larger and heavier rocks by sliding and rolling along the substratum. Traction occurs as a water erosion by river water. It results in the movement or transportation of large rocks or stones which are otherwise settled at the bottom, by the force exerted by the water current. These rocks or stones gets displaced from their sites and attains a rolling movement due to fast flowing water current.

the reason the two sea urchins don't compete with each other for food, resources, etc. is because the urchins have adapted to the environment but they have also adapted to each other. this is most likely because the genes of the sea urchins.