What occurs when a river's flow and level are affected by tides?

The correct answer is tidal. Tides are the regular rise and fall of sea levels caused by the gravitational forces exerted by the moon and the sun, as well as the rotation of the Earth. In areas where rivers meet the ocean, a phenomenon known as estuarine dynamics occurs. Here, the flow and level of the river can be significantly influenced by tidal actions, causing variations in the water level that can affect both the river's current and the freshwater it carries. This interaction creates a mixed environment where saltwater and freshwater meet, demonstrating the effect of tides on river systems.

The other options do not accurately describe this phenomenon. Freshwater typically refers to non-saline water in rivers and lakes, artificial relates to man-made changes to rivers, and seasonal indicates variations that occur over specific times of the year, which are not directly caused by tidal influences.