DISTURBANCE AND FISH COMMUNITIES IN INTERMITTENT TRIBUTARIES OF A WESTERN GREAT-PLAINS RIVER

Intermittent canyon tributaries of the Purgatoire River, Colorado, consist of isolated pools during long periods of low or no flow, which are punctuated during summer by intense flash floods lasting one to three days. Despite the frequent strong disturbances and limited opportunities for recolonization, we found fish in most permanent pools sampled along five tributaries. Of 11 native fishes in the river mainstem, five species penetrated an average of 6.6-9.1 km upstream in four drier tributaries; five others colonized only 0.3-1.0 km upstream; and one species was rare. Distribution of five species was different in the fifth tributary, which had more flow. Analysis of fish communities at sites sampled through time in four tributaries indicated that species composition and relative abundances remained relatively constant at three sites with deep complex pools but changed markedly at two sites with shallow simple habitat. This was probably because the latter offered little refuge from floods. We also found relatively variable species composition among pools along each tributary, little of which could be accounted for by drought, predation, or habitat complexity alone. We propose that differential effects of unpredictable floods in pools of varying habitat complexity, interacting with recolonization and recruitment, are sufficient to produce the spatial and temporal variation observed in tributary fish communities.