WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

ecosystem group and herbaceous productivity in the
forest ecosystems were eliminated entirely. Implica-
tions of this are discussed more fully in Chapter III.
One of the original proposals, assessment of the
impact of cloud seeding on avalanche activity, was
modified and resuQmitted and became the basis of a
separate contract handled directly by INSTAAR, the
final report of which will appear independently
(Armstrong et al. 1976).

Alpine Tundra Ecosystem
and Paleoecology
(Team Leader: CU)
Steering committee~ ~

Bureau of ~ Project Coordinator

ReClamati~n/ t

Forest Ecosystems and
Consultants Silver Studies
(Team Leader: CSU/FLC)

TUNDRA PROCESS STUDIES

Plant Productivity
Phenology
Boreal Toad Monitoring
Tundra Animal Studies
Geomorphic Processes

GENERAL & HISTORICAL STUDIES
Dendrochronology
Historical Climatology
Ecologic Overview

Silver Disposition
Silver Effects

FOREST PROCESS STUDIES

Phenology
Phytosociology
Tree Biomass
7ree Mo}sture Stress
Elk Studies
Small Mimmals

Figure 2. Projects and administrative organization of the San Juan Ecology Project.

DESCRIPTION OF THE STUDY AREA

Geology and Physiography

The elevation of the rugged San Juan Mountains
ranges from about 2000 m in some valleys to over
4200 m, with 14 peaks exceeding this latter eleva-
tion. The San Juan Mountains are essentially the
product of domal uplifting centered in the Needle
Mountains. Tectonic movements and accompanying
periods of intensive erosion have occurred repeatedly
throughout geological time. The intermittent uplift-
ing of the San Juan dome has been locally complicated
by faulting and volcanic activity. There were epi-
sodes of intensive volcanic activity, and only part
of the region southwest of the Needle Mountains
failed to receive a thick cover of volcanic material.

Erosion and uplifting continued after the volcanic
activity. The primary geomorphic agents most influ-
ential in creating the present distinctive form of
the mountains are running water, glaciers and, at
higher elevations, the collective group of denudational
processes known as "cold climate processes" including
frost action and solifluction. Climatic conditions
suitable for the generation of large valley glaciers
have recurred at regular intervals during the last 2
million years. Since the last glaciation ended 10 to
11 thousand years ago, climatic fluctuations of
smaller amplitude have been accompanied by fluctua-
tions in erosion (Andrews, this vol., p. 87). For a
more detailed geomorphic description of the San Juan
Mountains refer to Cross and Larsen (1935) and
Mather (1957).

Soils

Two soil classification projects have been undertaken
by the U.S. Forest Service. A broad survey (U.S.
Forest Service 1968) classifies the area on the basis
of both vegetative cover and soil type. The second,
more detailed soil survey (U.S. Forest Service 1965)
includes descriptions of the soil, along with slope
and erosion indexes. Generally, the tundra soils of
the San Juans can be classed as ~mmature, frequently
mobile or unstable, and subject to vertical and hori-

zontal response to frost action. Below timerline
the soils and slopes, except where invaded by aval-
anche chutes, talus cones, and landslides, are more
stable and mature under the protection of extensive
forests. Above 2900 m elevation the soils are clas-
sified as Inceptisols. These soils are loamy, or
loamy skeletal, and can be divided into Cryochrepts
and Cryumbrepts. The Cryochrep.ts occur under timber
vegetation and Cryumbrepts occur under grass, forb,
or shrub vegetation both in the tundra and below
timberline. Permeability class and water transmis-
sion rates of these soils are high.

Climate and Hydrology

Long term weather data are available from Durango,
Telluride, and Silverton. Climatic data for the
region have been compiled by the U.S. Department of
Commerce (1961), Grant (1969), Kuo and Cox (1975),
and Barry and Bradley (this vol., p. 43). The
climate can be classified as subalpine and alpine
continental. Annual precipitation ranges from approx-
imately 500 mm at lower elevations to as much as 1500
mm near the Continental Divide. The major moisture
source during winter is the Pacific Ocean. Heaviest
precipitation occurs when airflow moves from the
warmer Pacific water of southern California to the
mountains. Winter snowfall is moderate and on the
average contributes 54 percent of the annual precipi-
tation (Kuo and Cox 1975). However, naturally high
variation, approximately 60 to 170 percent of the
long term mean, exists between years. Winter precip-
itation also varies over an elevational gradient,
e.g., the'number of hours of winter precipitation in
1969-70 was 938 near the crest of the San Juan Moun-
tains versus only 152 at Durango. Littleredisposi-
tion of snow occurs in the lower timbered elevations,
while secondary movement of snow by wind in the
tundra causes a mosaic of exposed ground and drifts.
The main contribution to snowmelt runoff comes from
the elevation range between 3200 m and 3800 m. The
large elvation range prevents melt from occurring
over the total vertical snowpack profile at anyone
time. Summers are cool to mild with extensive,

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