The phrase "at flood" typically refers to a situation where a body of water is overflowing its banks or containment. There are many different ways to describe this state using synonyms. For example, you could say that the water is at flood stage, or that it is spilling over, flooding, or inundating the surrounding area. Alternatively, you might say that the water is cresting, breaching its banks, or causing a deluge. Each of these synonyms conveys a different shade of meaning, emphasizing aspects such as the height of the water, the degree of its spread, or the force with which it is flowing.