answer:Your “problem” has too many contradictions. You say the wall is rigid, but then it leans. Does it lean toward the water or away from it? Is the ground level or slanted? Are you saying the cow (cattle is plural, but you seem to be talking about only one animal) is leaning against the wall as the water rises? Once the water rises, the cow would not be leaning against the wall, it would be swimming. F=mA. There is very little acceleration, but how much mass for the cow? The cow would cause more force on the wall by swimming away while tied to the top. But the stronger force of the current in the river would create the most force. The cow will drown.