In dry weather it may not be possible to see the water flowing, or see any water at all. In streams with gravel, sandy or rock bottoms, the water may be flowing beneath or between the rocks. In biology, a stream is flowing if there is enough water to support aquatic life, including fish and gill-breathing amphibians, benthic insects, crustaceans, and mollusks. These can live in a shallow flow beneath rocks or logs. The flow may be so small that it can not be measured by stream flow measurement instruments.