new system : water currents

In the river biome, there would be currents going down a certain direction. The fish would go down the current and other strong fish could go against it. when swimming or in a boat it would be quicker going down the current than up. This could lead to water falls or if you walk up the current you could find an ocean or lake. items would go down the current. only can be found naturally and can be walled off by placing blocks like a dam