A tale of two rivers: historical and current status of fish, amphibian, and reptile species in the Santa Cruz River, Arizona.

Authors: Michael Bogan, Claire Zugmeyer, Peter Reinthal, Matthew Mayer

Year: 2025

Abstract

The Santa Cruz River is a major tributary of the Gila River in southern Arizona that has experienced dramatic habitat loss and alteration over the last 120 years. Historically, there were multiple perennial reaches on the mainstem of the river, interspersed with intermittent and ephemeral reaches, the combination of which supported a rich aquatic vertebrate fauna. Our goal here is to document historical records of fish, amphibian, and reptile species from the mainstem Santa Cruz River and contrast those records with modern-day species distributions. The historical (pre-1940) fauna included 6 native and 1 non-native species of fishes, 8 amphibian species, and 3 aquatic reptile species. With the exception of small parts of its headwaters, all perennial reaches of the river were dry by 1940 due to water withdrawals. However, treated effluent from wastewater treatment plants has restored flow in some of these reaches since the 1970s. Additionally, Tucson city managers have added flow to two additional reaches since 2019. Currently, 1 of the historical fish species is extinct (Santa Cruz Pupfish [Cyprinodon arcuatus]), 3 are likely extirpated from the mainstem river in Arizona, and 2 are extant (Gila Topminnow [Poeciliopsis occidentalis] and Longfin Dace [Agosia chrysogaster]). Amphibians and reptiles have fared better, with 7 of 8 amphibians and all 3 aquatic reptiles extant, but with greatly reduced distributions. The modern fauna of the river also includes 7 non-native fish species, 1 hybridogenetic fish species of unknown origin (Poeciliopsis monacha-occidentalis), and at least 3 non-native amphibians and reptile species. Additional surveys are needed in the headwaters of the river, where other native species may yet persist, and annual surveys should continue to monitor for non-native species in the rest of the mainstem. Recent reintroduction efforts (e.g. Gila Topminnow and Sonora Mud Turtle [Kinosternon sonoriense]) in downtown Tucson) have proven successful, and continued close collaboration between biologists and water managers could create additional restoration opportunities.