LOMA VERDE

WASH BASIN

Saguaro National

1989 Chiva fire

1999 Box Canyon Fire

1994

Mothers

Day Fire

0

0

1 MILE

1 KILOMETER

2003

Helens ll

fire

1994

Rincon

fire

MICA MOUNTAIN

JO

C

WILDHORSE

A

ANY

QU

O

I

N

CANYON

N

Park

110°45’ 32°15’

10’

32°5’

40’ 35’ 110°30’

Figure 1.  Saguaro National Park (black boundary) and extent of major fires occurring since 1989.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3140

January 2006

National Park Service

Effects of Post-Wildfire Sedimentation on 

Leopard Frog Habitat in Saguaro National Park

Printed on recycled paper

On June 18, 1999, a lightning 

strike in the torrid desert 

mountains of Saguaro 

National Park’s Rincon 

District east of Tucson, Arizona (fig. 1), 

ignited a blaze that, over the next three 

weeks, would burn 6,500 acres of dry 

scrub and oak and pine forest. The most 

immediate and most obvious effect of 

the Box Canyon fire was the blackening of hillslopes, including the charring 

of hundreds of saguaro cacti, the giant 

succulent for which the park is named. 

The fire, however, also caused changes 

to the hydrologic system that would soon 

threaten other park resources, including the habitat of the increasingly scarce 

lowland leopard frog (fig. 2).

In Loma Verde Wash, an ephemeral 

stream that drains the northwest corner of 

the park, small basins within the bedrock 

channel bottoms collect rainwater and, 

in some cases, spring water that provide 

habitat for the leopard frog. Pools, known 

as tinajas, that contain water year round 

are of particular importance to the frogs’ 

survival because they provide winter 

habitat for leopard frog tadpoles.

Runoff from the first rains following the Box Canyon fire carried large 

amounts of ash to ephemeral stream 

channels of Loma Verde Wash that 

left the waters in the pools inky black 

and greatly raised water temperatures. 

Although the stew of chemicals found 

in wildfire residue may have had some 

detrimental effects on the leopard frogs 

and other aquatic creatures, subsequent 

flows flushed the ash from the pools and 

a breeding population continued in the 

wash. Less than 2 years after the fire, 

however, a slug of gravel and coarse sand 

had made its way from the watershed 

slopes and headwaters into the uppermost 

pools, burying them to depths of 3 to 6 

feet (fig. 3). Within 3 years after the fire, 

all but a few of the 32 pools in which 

leopard frogs had been observed were 

buried in sediment. All 24 pools that had 

been identified as breeding pools were 

buried and remained so as of summer 

2005.

Habitat destruction and the introduction of non-native species throughout 

the Tucson Basin have reduced the 

once-abundant leopard frogs’ 

range to a few isolated canyons, 

including several in Saguaro 

National Park. This species has 

likewise been extirpated from 

many other areas in Arizona 

and from approximately half 

its range in the United States. 

The wide-scale disappearance 

of the lowland leopard frog 

has led to its classification as 

a Priority Vulnerable Species 

in the Sonoran Desert Conservation Plan implemented by 

Pima County in Arizona and 

as a Species of Special Concern by the 

Arizona Game and Fish Department. 

Lowland leopard frogs were also selected 

as one of three indicators of ecosystem 

health for Saguaro National Park. 

The destruction of habitat throughout 

Loma Verde Wash alerted park bioloFigure 2.  The lowland leopard frog’s population in

Saguaro National Park Rincon District is dependent on

perennial pools of water in bedrock basins.

Photograph by Erik Enderson, Tucson, Arizona
gists to the hazard presented by excessive 

sedimentation in the wake of wildfires. 

While conducting inventories of aquatic 

habitat, park biologists also discovered 

that a large pool in Wildhorse Canyon 

Wash, downstream from the 1989 Chiva 

fire, had filled with sediment at some 

unknown date. Sixteen years after the 

fire, the pool remains filled.

Wildfire Management

These discoveries have important 

implications for Park Service fire management policies, which are intended 

to establish a natural fire regimen in 

Saguaro National Park by prescribed 

burning and by selective suppression of 

naturally caused fires. Fire is a critical component of the dry oak and pine 

forests of Arizona. Frequent, naturally 

caused fires keep forest floors clear of 

heavy accumulations of woody vegetation, leaf and pine litter, dead trees and 

branches, and of overly dense forest 

stands, all of which can otherwise be fuel 

sources for much larger, stand-destroying 

fires. Fire management practices of the 

20th century, which involved suppression of most fires throughout the Western 

United States, have left many forests with 

high fuel loads that make them susceptible to intense and extensive runaway 

wildfires. 

Within Saguaro National Park’s 

Rincon District, a combination of factors, 

including past fire-management practices and high production of fuels during 

wet years followed by exceptionally dry 

years, has resulted in five hot, destructive wildfires covering about 40,000 

acres in the past 16 years (fig. 1). Burn 

areas typically result in higher volumes 

of storm runoff compared to runoff in the 

same areas during pre-fire conditions. 

Higher stream runoff is accompanied by 

widespread erosion in upland areas and 

deposition of exceptional volumes of 

sediment in lowland areas. 

The relation between wildfires and 

excessive sedimentation and the associated threat to the leopard frog can bring 

conflicting pressures on National Park 

Service fire management policies. Park 

managers are faced with the task of trying 

to reestablish and maintain a healthy, firedependent ecosystem at the same time 

they are trying to protect a dwindling 

population of frogs.

Sedimentation Study 

Objectives

To assist managers in the assessment 

of appropriate fire management practices 

for protecting aquatic habitat, the U.S. 

Geological Survey, in cooperation with 

the National Park Service, is conducting 

a study of hydrologic changes caused 

by uncontrolled wildfires and the effects 

of increased sediment transport and 

deposition on leopard frog habitat in the 

Rincon District of Saguaro National Park. 

Objectives of the 3-year project include: 

(1) estimate the background rates of 

sedimentation in perennial bedrock pools 

within the study area and characterize the 

sediments in pools that presently provide 

adequate habitat for the leopard frog and 

other aquatic species as well as in pools 

that have been rendered unfit for habitat 

because of excessive sedimentation, (2) 

determine mechanisms of sediment delivery from burned areas and the change in 

sediment yields caused by burning of the 

watersheds, (3) determine source areas 

of excess sedimentation in burned areas 

and their physical characteristics, such as 

topography, geology, soil type and thickness, and effects of fire on soil properties, and (4) estimate potential sediment 

yield from unburned areas in the event of 

future uncontrolled fires. 

Physical Setting 

The Rincon District of Saguaro 

National Park encompasses about 105 

square miles of rugged upland desert and 

mountainous terrain east of Tucson. Altitude ranges from about 2,500 feet along 

the western edge of the park to about 

8,500 feet on several peaks in the Rincon 

Mountains. Most of the park is mountainous, but the most heavily visited areas are 

bajadas, landscape features formed by 

overlapping alluvial fans, along the base 

of the mountains at elevations between 

about 2,500 and 3,200 feet. The climate 

is semiarid with summer and winter rainy 

seasons. Average annual precipitation is 

about 12 inches per year near the base 

of the mountains and about 28 inches 

per year at the highest elevations, where 

some precipitation falls as snow.

The bajadas of the lower elevations are 

blanketed with Sonoran Desert vegetation, which includes extensive stands of 

saguaro cactus as well as an abundance of 

other cacti. With increasing elevation, the 

hillslopes are vegetated with grassland 

and scrubland, oak and juniper woodlands, juniper and piñon pine woodlands, 

Ponderosa pine forest, and mixed conifer 

forest. 

The rocks of the Rincon Mountains 

are predominantly granitic, in many 

cases highly fractured and sheared, 

which weather to coarse, gravely sand 

that covers most hillslopes although 

steep slopes of exposed bedrock are 

common. The mountains are dissected 

by a dense drainage system of small, 

steep-gradient streams, most of which 

are ephemeral, flowing only in response 

to storms or periods of runoff. Stream 

channels change dramatically in physical 

characteristics as they cross over terrain of varying steepness and geology. 

Where channels drain slopes covered 

with unconsolidated materials, such as on 

soil-covered hillslopes and through the 

alluvial fan deposits of the bajadas, the 

channel banks typically are composed of 

uncemented to moderately cemented sand 

and gravel. 

Many channel reaches cross outcrops 

of resistant bedrock. Tinajas, are found in 

these bedrock reaches and provide habitat 

for the lowland leopard frog. Tinajas 

range from about 3 to 30 feet in diameter.

Figure 3.  A tinaja in Loma Verde Wash before (left) and several months after (right) the Box

Canyon fire of 1999. Within 2 years of the fire, all of the tinajas downstream from this one had filled

with sand.

Photographs by Don Swann, Saguaro National Park, 1997 and 1999
swiftest, and fine sand and silt typically 

are deposited in quieter environments, 

such as flood plains or backwater pools. 

Under normal conditions, channels in the 

Rincon Mountains have little sediment in 

storage, and in steep reaches where they 

cut into solid bedrock there can be almost 

none.

Large, hot wildfires, however, alter 

watershed conditions in ways that can 

greatly increase the rate of sediment 

delivery to channels. The extensive 

destruction of vegetation from grasses 

to trees, as well as the layer of decaying 

leaves and conifer needles that comprise 

the forest litter, exposes the ground 

surface to direct effects of precipitation 

and storm runoff. Fires can burn into 

root systems that ordinarily bind soil to 

hillslopes, and finally, hot fires change 

the structure of the soil itself, making it 

resistant to infiltration from precipitation.

Such watershed changes accelerate the 

delivery of sediment to stream channels. 

The lack of vegetative cover and change 

in soil structure increase the amount of 

surface runoff in response to storms, 

resulting in fast-moving water with 

increased ability to erode and transport 

sediment on hillslopes. The low permeability of the soil and lack of a forest 

Figure 5.  Mixed conifer forest on the north slope of

Mica Mountain was destroyed by the Helens II fire in

2003.

Fire History

From 1943 through 2003, at least 40 

fires affected over 57,000 acres in and 

adjacent to Saguaro National Park lands 

in the Rincon Mountains. Just six fires, 

however, burned about 92 percent of that 

total, and nearly 75 percent of the burned 

acreage was caused by five wildfires after 

1988 (fig. 4). Of those recent fires, the 

Rincon fire of 1994 was by far the largest 

historical fire, burning about 23,300 acres 

in the park and in Coronado National 

Forest lands surrounding the park. 

Burned acreage is based on the perimeter 

of the fire area and may include sizeable 

swaths of vegetation that are untouched 

by fire as well as many individual trees 

and other plants that are lightly burned 

and survive.

Fires have swept through nearly 40 

percent of the park’s forests, grasslands, 

and scrublands since 1989, burning about 

50 percent of the forested areas and 35 

percent of chaparral and desert scrublands and grasslands. The Chiva, Mothers 

Day, and Box Canyon fires accounted for 

most of the burned acreage in scrublands 

and grasslands, and the Rincon and Helens II fires primarily burned upper elevation forests and woodlands. Nearly the 

entire 1,000 acres of mixed conifer forest 

on Mica Mountain, the smallest vegetation zone in the park, was affected by the 

Helens II fire of 2003 (fig. 1). The north 

slope of Mica Mountain was particularly hard hit by the fire, which wreaked 

near-total destruction on many stands of 

Ponderosa pine and other conifers (fig. 5); 

however, other stands within the fire area 

were virtually untouched and many trees 

survived the fire despite charring of trunks 

and branches.

Fire and Hydrology

 Sediment moves down hillslopes 

to enter stream channels by slow 

processes, such as soil creep in 

which sediment particles move gradually downhill under the force of 

gravity, or by faster processes, such 

as rill erosion or sheetwash in which 

sediment particles are entrained and 

transported by water flowing over 

the ground surface. 

Once in the channel, sediment is 

moved downstream by streamflow. 

Along its course from upland basins 

to lowland valleys, sediment may be 

deposited as sand and gravel bars, 

on flood plains, or on the channel 

bottom. Such sediment is considered 

to be in storage and may be so for a 

period of hours to centuries depending on location and the magnitude of 

future streamflows. 

A stream’s ability to transport 

sediment depends on the strength of 

the streamflow and the size of the 

sediment particles. Coarse particles, 

such as pebbles, cobbles, and boulders, typically are deposited on the 

channel bottom where flows are 

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Figure 4.  Fire history of Rincon District of Saguaro National Park since 1943.
canopy to impede rainfall cause water to 

accumulate on the ground surface. This 

water then moves downstream as sheets 

of water (sheetwash erosion) or gathers in 

small streams that form rills and gullies. 

Under normal conditions, sheetwash and 

rill erosion are not significant processes 

on forested hillslopes, but after intense 

fires, the frequency and magnitude of such 

processes caused by watershed changes 

can persist for several years.

The increased transport of sediment 

from burned hillslopes can overwhelm the 

sediment-carrying capacity of a stream, 

resulting in excessive deposition of fresh 

sediment in stream channels. The amount 

of time the sediment remains stored in the 

channels depends on the volume of sediment available on hillslopes for transport, 

the period of time that is required for the 

conditions to stabilize on hillslopes, the 

frequency and size of streamflows available for scouring the sediment from the 

channel, and the physical characteristics 

of stream channels that govern the velocity of flows through a particular channel 

reach. 

Study Approaches

The principal tasks needed to meet the 

objectives of the USGS study include a 

watershed analysis of the park’s drainage basins, analysis of historical data, 

particularly photographs, field observations of erosion and sediment-transport 

mechanisms, and the changes in watershed conditions over the course of the 

study, and application of sediment erosion 

and transport rates in post-fire conditions 

determined from studies in other mountainous regions. 

The watershed analysis involves the 

compilation of spatial data that are relevant to sediment transport. These include 

hillslope gradient, maximum elevation 

difference, channel density (total channel 

length per area), channel gradient, aspect, 

soil cover, vegetation cover, fire history, 

and fuel load. The analysis then is used to 

create an index of critical sediment-source 

areas most likely to produce excessive 

sedimentation in stream channels below 

the watershed. 

Field observations include the mapping, photographing, and measurement 

of erosional features in recent burn areas 

and comparison with watershed conditions 

in older burn areas as well as mapping 

of sediment movement and storage in 

stream channels below burn areas. Sediment samples were collected from several 

depositional environments for particlesize analysis to assess the changes in 

sediment transport energies occurring 

in post-fire conditions on hillslopes and 

stream channels. Field observations and 

measurements of stream-channel physical 

properties are used to evaluate those pools 

that are most likely to be buried during 

episodes of extreme sedimentation and 

most likely to remain buried for sustained 

periods. 

Recent Observations

The Helens II fire of 2003 occurred 

1 year before the onset of this study 

and provided an opportunity to observe 

watershed conditions in the first several 

years following a major fire. The first 

field observations in the Helens II burn 

area were made in June 2004. Although 

summer-monsoon and winter precipitation 

had been below average following the fire, 

evidence of greater than normal sediment 

movement on the severely burned north 

hillslopes was 

plentiful, particularly from sheet 

wash erosion and 

accelerated soil 

creep. Sediment 

accumulation 

in the upstream 

reaches of channels appeared to 

be considerably 

greater in those 

channels below 

the burn area 

than in channels 

below unburned 

or lightly burned 

areas.

In the lower 

reaches of 

Joaquin Canyon 

north of the park boundary (fig. 1), ashy, 

fine-grained sediment that was flushed 

from the burned slopes in the first rains 

following the fire had been deposited on 

the channel bed and in bedrock pools. 

Although the bottoms and shoreline of 

some of the larger, water-filled pools were 

covered with black, ash-laden sediment, 

they still contained aquatic insects. 

Following the light 2004 monsoon 

season, little change was observed in the 

Helens II burn area and in the channels 

of Joaquin Canyon below the burn area. 

The next visit took place in July 2005, 

following a slightly wetter than average 

winter. Fresh rills and gullies were noted 

on some of the steeper slopes in the burn 

area as well as small debris flows on some 

slopes. Debris flows are mass movements 

consisting of a slurry of water and sediment in which the sediment transported 

can range from the smallest clay, silt, 

and sand particles to boulders. On other 

slopes, however, dense stands of bracken 

fern appear to have begun stabilizing 

areas that had been subject to sheetwash 

erosion and some rill development in the 

first year after the fire (fig. 6).

In the Joaquin Canyon, meanwhile, 

at least one large pool was buried by as 

much as 3 feet of coarse sand between 

field visits made in September 2004 and 

June 2005. Pools downstream from the 

filled pool had received some coarse-sand 

deposition but were not yet filled.

Final field observations for this project 

were to be completed in fall 2005. The 

project will conclude by fall 2006.

— John T.C. Parker

For additional information, please 

contact:

John T.C. Parker (jtparker@usgs.gov)

U.S. Geological Survey

520 North Park Avenue

Suite 221

Tucson, AZ 85719–5035

520-670-6671

Figure 6.  Bracken fern in the Helens II burn area on Mica Mountain has

begun to stabilize soils on hillslopes two years after the fire, July 2005.