FLOOD09816

• County: Statewide
• Basin: Statewide
• Title: Forest- Fire Devegetation and Drainage Basin Adjustments in Mountenous Terrain
• Date: 1/1/1977
• Prepared By: Department of Geology University of New Mexico
• Floodplain - Doc Type: Educational/Technical/Reference Information

W. D. White and $. G. Wells

the stabilization of the trunk streams.

INTRODUCTION

Disturbances in the natural state of a drainage basin due to changes in ve-
getation commonly affect erosion and sedimentation of hillslopes and stream
channels. The effects of vegetational change on hydrologic and geologic sys-
tems have been investigated for durations of geolofic time (Schumm, 1968) and
for shorter, historical periods (Dodge, 1902; Rich, 1911: Anderson and Tro-
bitz, 1949; Storey et aL, 1964: Dryness. 1965; Harris. 1977). The recent
geomorphic studies deal primarily ;""hh sediment yield and runoff changes in
response to land-use changes, such as logging. mining or devegetation
(Harris, 1977: Brater, 1939). Catastrophic ecologic events, such as forest
fires, can trigger complex geomorphic responses as portions of drainage
basins react to changes in sediment source areas and to changes in sediment
yields from these areas. This paper deals with forest-fire devegetation and
geomorphic adjustments of small drainage basins in the Jemez Mountains,
northcentral New Mexico (fig. 1). In June 1977, the La Mesa fire burned
approximately 59.5 km2 of forested land of whh;:h 41.5 km2 are included in
Bandelier Natio~al Monument. This forest fire presented an opportunity to
evaluate quantitatively the types of fluvial adjustments made by small, head-
water drainage basins to post..fire conditions.

The purposes of this paper are to (1) determine controls and rates of
sediment supply to fluvial systems over short time periods, (2) delineate
so~rce areas for sediment contributed to fluvial systems, (3) determine the
types of fluvial.adjustments in response to accelerate~ erosion, and (4) com-
pare established models of fluvial adjustments in undisturbed drainage basins
to watersheds which differ in the amount and extent of fire_ devegetation,
hereafter called burn intensity.

STUDY AREAS

To accomplish these objectives, measuring instruments were placed in six
drainage basins of the Frijoles Canyon watershed (fig. 1), three months after
the fire. The basins are located in two areas which differ in veg~!ation and
climatic settings. The Burnt Mesa study area'is at an elevation of approxi-
mately 2195 m and is characterized by a mosaic of Ponderosa p~ne (Pinus
ponderosa) in dense young stands, open park like stands of mature trees, and
open meadows of grasses. The Apache Springs area, at an elevation of about
.2530 m, is a mixed-conifer fOl.'est comprised of Ponderosa pine (~ ponde-
rosa), White fir (Abies concolor), Douglas-fir (Pseudotsu~a menzieisii) ~ and
aspen (Populus. tremuloides). Precipitation in the Burnt Mesa area for Octo-
ber 1977-November 1978 was about 825 mm. Most of the precipitation fen
during the winter and spring. . No a.ccurate precipitation records exist for
the Apache Springs area. '

In each study area three low-order drainage basins were selected on the
basis of the degree of burn intensity:

intense = complete destruction of all ground and tree foliage, leaving
only charred trunks and branches (fig. 2A);

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