Pure water at room temperature or 25 degree Celsius has a pH very close to 7. This is known as neutral pH.
An acid has a pH lower than 7 with the pH decreasing as the concentration of the acid increases. On the other hand for a base the pH is greater than 7 with the pH increasing as the concentration of the base increases.
After an acid spill, the pH of the river is sure to drop to a value below 7 which is the neutral pH or that of water. As the river flows, the movement of water will slowly replace the acidified water with fresh water. The pH too will gradually increase and reach that of pure water. This is the neutral pH of 7.
What you have to determine is how long it took for the fresh water to replace the acidified water and for the pH to return to that of 7.