Post-Wildfire Sedimentation in Saguaro National Park, Rincon Mountain District, and Effects on Lowland Leopard Frog Habitat

Abstract

The Rincon Mountain District of Saguaro National Park occupies about 272 square kilometers of mountains, canyons, and alluvial fans in southeastern Arizona just east of Tucson. The park contains some of the last remaining habitat in the Tucson Basin of the lowland leopard frog that lives in the bedrock pools called tinajas in canyons at elevations between 850 and 1,800 meters. Those tinajas that contain water year-round are critical winter habitat for tadpoles, and the breeding success of the leopard frogs depends on these features. In recent years, many tinajas that previously had provided habitat for the leopard frogs have been buried beneath large volumes of coarse sandy gravel that resulted from severe, stand-replacing wildfires in the watersheds above them. The U. S. Geological Survey in cooperation with the National Park Service, conducted a study in 2004-06 to determine critical sediment-source areas, and the mechanisms of sediment delivery from hillslopes to stream channels to areas of leopard frog habitat and to estimate the increase in rates of sedimentation resulting from wildfires. Spatial data of watershed characteristics, as well as historical data, including photographs, monitoring surveys, precipitation and stream discharge records, were used in conjunction with field observations conducted between spring 2004 and fall 2005. The Helens II fire in 2003, the fifth largest wildfire to burn in the Rincon Mountains since 1989, offered an opportunity to observe mechanisms of sediment erosion, transport, and deposition in the immediate post-fire environment. Reduction of the forest canopy, understory vegetation, and organic litter on the ground surface in severe burn areas caused increased surface runoff in the Joaquin Canyon watershed that led to intensified erosion of hillslopes. An initial flush of fine material, mostly ash, was transported to lower channel reaches with the first significant precipitation event following the fire. Subsequently, the main erosional mechanisms were rainsplash and sheetwash that delivered high sediment loads to headwater tributaries. The increased runoff also led to scouring of the headwater tributaries and the downstream transport of a sediment slug by a series of episodic debris flows or hyperconcentrated flows. The sediment slug, following intense summer precipitation, moved downstream several hundred meters at a time. Sediment was remobilized during subsequent periods of runoff. As of fall 2005, sediment had traveled 3.3 km downstream from the nearest burn area margin and had buried several tinajas in as much as a meter of sediment. Sediment continued to overwhelm the transport capacity of the channel even as the hillslopes in the burn area were showing evidence of recovery. The sedimentation history and effects on leopard frog habitat in other channels in the Rincon Mountains was evaluated by analyzing observations made by Saguaro National Park staff during monitoring surveys of leopard frog populations. The best record of post-wildfire sediment deposition was that of Loma Verde Wash in which the filling of all tinajas in the two years after the 1999 Box Canyon fire was recorded. Monitoring of leopard frog populations in Wildhorse Canyon appeared to reflect the lingering effects of heavy sedimentation related to the 1989 Chiva fire. Populations appear to be recovering in the upper tinajas, which were mainly free of sediment, but sightings of frogs were sparse in the lower tinajas that still contained high volumes of sediment. In Madrona Canyon, leopard frog sightings were sparse, possibly indicating that habitat had been detrimentally affected by the Rincon fire of 1994. Based on rates of filling of tinajas in Joaquin Canyon and Loma Verde Wash, minimum estimated rates of sediment yield from burn areas ranged from 425 to 1,960 kg ha-1. The residence time of sediment in tinajas was found to be highly variable. Tinajas in Loma Verde Wash that were buried following the