Relationship between U.S.-Mexico border operations and total suspended sediment in headwaters of the Rio Yaqui.

Authors: Stephani L. Clark, Scott A. Bonar

Year: 2012 (xliv)

Abstract

Impacts of U.S./Mexico border operations on many terrestrial species have been well documented, yet little is known about the impacts of border operations on aquatic ecosystems. Black Draw, a tributary to the headwaters of the Rio Yaqui, originates in San Bernardino National Wildlife Refuge (SBNWR) on the United States (U.S.)- Mexico border and is home to several species of threatened or endangered fishes. Construction of bridges and roads on or near SBNWR by U.S. Border Patrol, and other activities conducted by the U.S. Border Patrol, such as dragging roads to check for footprints and patrolling dirt roads in and around SBNWR, have prompted concern about increased sedimentation of the Rio Yaqui. Increased sedimentation could elevate total suspended sediment (TSS) and turbidity in the Rio Yaqui thus causing changes in diet and survival of fishes inhabiting the system. To evaluate impact of border patrol activities on TSS and turbidity in Black Draw, we installed Nalgene stormwater samplers, which collect single 1L first flush grab samples of storm water. Samplers were installed at randomly chosen sites, above and below three areas of border operations: The US/Mexico Border at Black Draw; The US/Mexico Border at Hay Hollow Wash; and the Black Draw Crossing at Geronimo Trail, a road in the United States which is graded to facilitate discovery of illegal border activity. Samplers were also located at additional random sites throughout Black Draw to identify major sources of sediment to the stream. Initial results show TSS concentrations ranged from 3.0mg/L to 67340.4mg/L and turbidity ranged from 0.6 nephelometric turbidity units (NTU) to greater than 10,000NTU. Turbidity and TSS concentrations are now being plotted and compared among locations in the drainage. Furthermore, these values are being used to identify a range of turbidity and TSS concentrations for which to test effects on Rio Yaqui fishes in laboratory experiments. Results will be used to inform management considerations involving turbidity, TSS and the imperiled fishes of the Rio Yaqui drainage.