anything and everything in the water...the bottom, submerged stuff , etc- is going to create flow movement and patterns. ie the water is not going to just flow and run in a nice ribbon at the same speed. every obstacle is going to impact the water.  eddies, rip tides,  tree stumps, rocks, boulders, plants growth,  etc.  when the water goes around something it creates a swirl effect that can be compounded quickly to being a danger.  the tree stump caused the water to not just flow past-   then faster, swirling water created it own, dangerous, current.  it's the same principle i use in my common oven.  a cookie sheet on the bottom rack about 8 inches off the floor of the oven will cause the heated air coming from the element to swirl to get around it and soon the whole oven is a hot air in motion. you have a convection oven - it will cook "hotter" and faster - i turn down the suggested temp about 10 degrees.   exact same principal - water/air going past an obstacle creates deviation.