WMOD00597

• Project Name: Sierra Ecology Project
• Title: The Sierra Ecology Project - Volume II - Workshop IV, Workshop V
• Date: 7/1/1980
• Weather Modification - Doc Type: Report

General Physical Conditions

~

Geography . -The physiography of the project area is described in the preface to Volume I of
this report series, Ecological Description of the American River Basin (Smith and Huntington
[2]). However, several factors are relevant to the current discussion. The Sierra Nevada rises
gradually from west to east, approximating a 3- to 4-percent gradient between Folsom Dam and
the Sierra crest near Pyramid Peak. This is a gentle slope from a meteorological standpoint. On a
local scale, however, dissection of the main mountain block by steep-sided valleys results in gra-
dients of 10 to 40 percent for 94 percent of all slopes. From the standpoint of stream channel
development and soil erosion, the American River Basin is a high-energy site capable of abundant
geomorphic work. In addition to the generally steep hillslope configuration, 75 percent of the
total aspects face south and west. This generally "warm" site orientation has implications for
snowmelt rates, distribution of vegetation, and soil-forming processes.

..

..

Geology and Soils.-The soils of the 0.51 x 106-ha (126-million-acre) American River water-
shed reflect the area's geologic, vegetational, and climatic diversity. The geologic history of the
Sierra includes metamorphism of sedimentary rocks, volcanic eruptions, intrusions of igneous
magma, and glaciation. Each of these processes has contributed to the existing soil constituency.
Superimposed on these different parent materials are differences in topography, time, vegeta-
tion, and climate. Each factor plays an important part in soil development in the watershed.
Perhaps the most important are time and climate. Vegetation varies from annual grasslands at
the lowest elevations to subalpine and alpine vegetation at the highest elevations. However, the
vegetation is primarily coniferous forest in the 1220- to 2440-m (4000- to 8ooo-ft) elevation zone,
where the major impact of precipitation increases as a result of weather modification will occur.
Topography affects the balance between soil formation and soil loss through erosion. It also
affects the amount of precipitation entering the soil. The topography of the watershed varies
from gently dipping ridge tops to nearly vertical canyon walls. Usually, shallow soils are found on
steep slopes and deeper soils on flatter slopes. This generalization may not apply, however,
because of the effect of parent material, climate, and time on soils.
Soil is formed by the chemical and physical weathering of minerals in the parent material. A
distinct vertical zonation of soils from any parent material may result from vertical changes in
precipitation and temperature in the watershed. As rainfall (not total precipitation) and mean an-
nual temperature increase, the rate of soil formation increases (fig. 1). The elevation range at
which maximum weathering and soil formation takes place on parent materials is shown in zone
A in figure 1. Time is important to soil formation because reaction rates are slow and relatively
long periods are needed to develop a soil. Thus, a 1O,000-year-old glacial deposit will have a less
developed soil than a similar deposit of 50,000 years ago. Because many of the lava flows are of
similar age and much of the granite in the American River Basin was exposed by erosion at similar
times, age differences in soils formed from hard rocks are not significant to this study.
Rocks and mineral deposits themselves also have a large effect upon soils. The very old, soft,
steeply dipping beds of metasedimentary rocks form predominantly shallow soils in the hot, low-
rainfall zones of the watershed.
Volcanic and metavolcanic rocks form a wide variety of soils in the main portion of the
American River Basin watershed. Rock hardness is a major determining factor of soil depth.
Granitic parent material and glacial deposits are found at higher elevations. The granitic rock
weathers deeply because it is porous compared to the hard volcanic rocks in the watershed. Soils
on granite are often thin because of climatic conditions and the lack of chemical weathering, but
the rock has physically decomposed to gruss at great depths.

~

IV-5