WordNet sense 1 (something that resembles a flowing stream in moving continuously):

flow

WordNet sense 2 (a natural body of running water flowing on or under the earth):

watercourse

WordNet sense 3 (dominant course (suggestive of running water) of successive events or ideas):

current

WordNet sense 4 (rain heavily):

pelt, pour, rain buckets, rain cats and dogs

WordNet sense 5 (flow freely and abundantly):

well out

WordNet sense 6 (move in large numbers):

swarm, teem, pullulate