What interferes with river flow?

There are several manmade and natural barriers that interferes with the river flow

Explanation:

The following structures man made structures interferes with the flow of a river -

a) Gyrones

b) Canyon

c) Dams

d) diverging canals

e) Bridge abutments/pillars

f) embankments

g) spill ways

h) drainage systems

etc

Along with manmade structure several natural obstruction also interferes with the river flow -

a) Undirected mearning

b) Catchment area slope

c) slope of the river itself

d) natural barriers like valleys, hills and mountains

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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.