A “thousand-year” flood provides long-term ephemeral habitat for “peripheral” fishes in the southern Sonoran Desert< Mexico

Authors: Lloyd T. Findley, Mauricia Pérez-Tello, W. Linn Montgomery

Year: 2010 (xlii)

Abstract

In earliest September 2009, large and slowly moving Tropical Storm Jimena “parked” itself over the central Gulf of California, bringing unprecedented torrential rains to surrounding coastal regions. During 36 hours of continual strong rains Jimena dumped 72 cm of water on the area around Guaymas, Sonora, causing severe flooding, much damage or loss of property, and extreme scouring of normally dry water courses in canyons and arroyos on the southern flank of the Sierra El Aguaje, just north of the seaside town of San Carlos. Based on extent of scouring and widening of arroyo beds, and the washing away of several Native American archaeological sites on former arroyo terraces, it seems appropriate to refer to Jimena as causing a “thousand-year” flood in the region. Scouring and collateral removal of established “riparian” vegetation was so severe as to place many normally dry or semi-wet water courses on ecological “re-set.” Following the flash-flooding (and physical destruction) wrought by the storm, several such arroyos continued to conduct unprecedented surface flows to the sea for periods ranging from many days to weeks. Even after several months, when their lower courses and surface connections to the sea had desiccated, the upper stretches of some arroyos with exposed sections of bedrock became long-term, albeit eventually ephemeral, running streams. On six occasions during spring 2010, we and several colleagues hiked to one such stream in the relatively small Arroyo Baviso, tributary to the larger, more sandy, and by-then-dry Arroyo Nacapule-San Carlos. There, we superficially surveyed the arroyo’s short and shallow stream and newly establishing “riparian” vegetation and collected aquatic organisms. During several weeks, the persisting surface flow and new stream-side vegetation imparted an impressive long-term aspect to the (surmised) eventual ephemeral condition of the stream. Although there are no “primary” freshwater fishes known from the surrounding area, the stream’s ephemeral connection to a small seaside estuary during the storm’s immediate aftermath allowed up-stream migration and temporary colonization of Arroyo Baviso by relatively large numbers of three species of notably euryhaline (often referred to a “peripheral”) fishes: Agonostomus monticola, mountain mullet, Mugilidae; Eleotris picta, spotted sleeper, Eleotridae; and Awaous transandeanus (perhaps A. banana), river goby, Gobiidae (all with voucher specimens). Also vouchered were two species of langostinos which had likewise penetrated the arroyo: Macrobrachium olfersi and M. sp. (awaiting identification). Additionally, two other “peripheral” euryhaline fishes, Dormitator latifrons (Pacific fat sleeper, Eleotridae) and Eucinostomus sp. (a mojarra, Gerreidae), were photographically documented in remnant large pools in the nearby and larger Arroyo El Palmar by Michael Bogan as he searched for aquatic insects. Thus, a total of at least five species of fishes and two species of langostinos have been added (at least temporally) to the region’s formerly almost non-existent “freshwater” fauna. At last visit to Arroyo Baviso in May 2010, the shallow surface flow was decreasing, but still running; an unprecedented nine months following the storm.