The monsoon delivers about one third of the annual 15-inch average precipitation in Arizona’s Ciénega Valley, for a few hours flooding pools and increasing streamflow up to a hundred times the normal rate through Ciénega Creek and Pantano Wash. The crest for a one-to-two inch rain brings a muddy torrent through Empire Gulch, roiling water, moving rocks and logs, picking up soil and debris. What strategy does the normally placid stream’s Chiracahua leopard frog use to survive the onslaught?
By Dos Aguas
Dos Aguas returned to Empire Gulch in August, an hour southeast of Tucson, Arizona, on the Empire Ranch, to see how high the recent monsoon waters reached, and how the storms had affected Empire Gulch’s frogs, one of a few dozen remaining populations of the Threatened Chiricahua leopard frog.
The Gulch that August morning, just steps away from the surrounding Sonoran Desert, was shady, cool, deserted, covered with a carpet of greenery, and sporting a muddy stream channel winding through its downed timber. It sits just above 4,000 feet elevation, and is dry most of the year except for a short middle section kept perennially wet by a small spring.
Below the spring the short sequence of perennial pools seemed much as they had before the monsoon, quiet, partially covered with surface algae, with a slow trickle of water sliding from pool to pool. The main difference from June to August was the appearance of many Chiricahua leopard tadpoles, feeding at the surface and darting about. Along the wide banks above the pools the long grasses were still bent flat on the ground from the heavy monsoon current two days before, but the tadpoles seemed not to have noticed.
The perennial water in Empire Gulch is a small flow, measured in tenths of a cubic feet per second (cfs), but steady enough to support the Chiricahua leopard frogs which live in its few pools.
The monsoon season delivers about one third of the annual fifteen-inch average precipitation in the Ciénega Valley. Some of this water goes to runoff, for a few hours increasing the streamflow up to a hundred times its normal rate. The crest for a one-to-two inch rain brings a muddy torrent through Empire Gulch, roiling water four to eight feet wide (instead of one foot or less) and three to five feet deep (instead of one to two inches). It moves rocks and logs, picks up soil and debris, and rushes northward through Ciénega Creek and Pantano Wash toward the Rillito River.
The first two monsoon rains in July, 2014, tempered the afternoon flows without producing a torrent, but from the third storm on each new rain created a short burst of heavy flooding. An inch of rain on August 1 pushed the flow from 0.39 cfs to 58 cfs.
This environment seems fragile – tiny in size, with a barely adequate water source (even in the presence of monsoons), and stark boundaries across which the heat and the aridity can be fatal to most of the life which thrives here. How do frogs, Threatened frogs, and the dozens of visibly healthy tadpoles survive in this place when their normally quiet pools one or two feet deep are overrun by yards-wide flood waters four or more feet deep?
Philip Rosen has some answers to those survival issues. Rosen is Assistant Research Scientist at the University of Arizona, who for more than two decades has made his professional focus “to understand the ecology and conservation biology of amphibians and reptiles in southwestern North America.” He has a long list of published studies, including turtles (the desert tortoise and the Southwestern mud turtle), snakes (the Mexican rosy boa and the Western shovel-nosed snake), lizards (the regal horned and the red-backed whiptail lizards), as well as several varieties of frogs, both native and non-native. Phil has a full beard, greying in patches, thinning hair, and a big smile. He loves to talk about native frogs.
Dos Aguas asked Phil a few novice questions about frog survival:
Can adult frogs in a stream like this sense a coming flood and leave the channel?
Although no one has ‘shown’ that adults can sense and avoid floods,there is no need to hedge on this.
If flood waters remove them from good habitat, can adult frogs find their way back?
Same for this statement. “Can” is the appropriate word for both. There can be no doubt. Frog census-takers have determined that adult leopard frogs can navigate over land to appropriate water, sometimes for several miles.
Do leopard frogs need floods to scour the channel and improve the habitat?
The idea that exceptional flood scour is required to sustain frog habitat in streams in general is a little off-base…excessive flood scour also can degrade habitat. The point is that summer flooding (and scour) is differentially detrimental to bullfrogs [a predator to leopard frogs], and thus a saving feature for leopard frogs.
Does breeding early in the season protect eggs and tadpoles from flooding risk?
This point applies to lowland leopard frogs especially (Rana yavapaiensis), although it is not demonstrated for the CLF [Chiricahua leopard frog]…so it isn’t a supportable statement at this point.
CLF Adapted to Survive
Whether by flight, overland journeys, or simply sheltering themselves in the deeper pools, CLF, as herpetologists refer to them, have developed a system to sustain life in this fragile ecosystem, even with young exposed to the summer floods. They have adapted to survive where their competitors cannot.
Dos Aguas followed the water farther east. Within a few hundred yards the pools disappeared, and then the flow itself was gone, sinking into the soggy ground in a patch of reeds. In another fifty yards by the ranch road crossing, where a slab of concrete has been laid over a small culvert, the ground was as dry on top as pre-monsoon. Only a few shorter cottonwoods survived there, and no frogs.
Dos Aguas hosts an online journal of water stories. For other stories see www.dosaguas.org/.