Tornadoes don't have a particularly significant influence. When tornadoes remove vegetation it may increase runoff and erosion near rives, increasing the amount of water and sediment that enters the river, but this is probably a small contribution. Tornadoes that cross rivers may suck unfortunate fish and other aquatic animals into the air as well. A tornado in Minnesota in 1939 was reported to have momentarily sucked the Mississippi river dry. This, however, was likely an exaggeration.
They can. Rivers are by no means a requirement for tornado formation, but neither do they prevent tornadoes in any way. A number of tornadoes, for example, have crossed the Mississippi river.
Yes, Valleys and rivers do not stop tornadoes. If tornadoes have occurred near your town or city there isn't really anything preventing a direct hit.
None. There is nothing that humans can do to stop tornadoes.
Nothing. Tornadoes are beyond our control; we cannot stop them. We can only prepare for them.
No, rivers do not affect tornadoes.
Nothing. It is impossible to stop tornadoes. We can only warn people and help them prepare.
Unfortunately, there is nothing that anybody can do to stop tornadoes. They are a force of nature and are entirely beyond our control.
No. The idea that hills stop tornadoes is a myth.
Nothing. There is nothing people can do to stop tornadoes.
No. It is impossible.
Yes, there have been a number of cases of tornadoes crossing rivers and lakes.
They can. The belief that hills stop tornadoes is a complete myth.