WSP04850

• File Number: 8541
• Description: San Luis Valley Project
• State: CO
• Basin: Rio Grande
• Water Division: 3
• Date: 2/11/1982
• Title: Draft Supplement to Final Environmental Statement
• Water Supply Pro - Doc Type: EIS

1

the land within project boundaries in 1977, which indicated about 40,000 acres
of wetlands were within the 138,500 acres of project-included lands.

1

More detailed studies indicated that between 26,000 and 47,000 acres of wetlands
and wetland habitat were present depending on the year the analysis was made.
The primary difference between these two wetland acreages was in the delineation
of ephemeral wetlands.

J

I

In 1978 the FWS initiated the National Wetlands Inventory for the San Luis
Valley and Closed Basin Division wetlands under a memorandum of agreement with
the Bureau. The inventory indicated that 43,122 acres of wetlands were present
within the proposed project boundary. These wetland acreages are similar to
those of the Bureau studies and have been accepted for use in this supplemdnt.

I

Wetlands and wetland habitat in some parts of the project will be affected where
existing ground water tables are less than 2-1k feet. Lowering of the water
table through pumping of the unconfined aquifer will alter existing vegetation.
An increase in loss of free-standing water due to seepage could also occur in
some areas.

I

I

Map overlays of the top of the unconfined aquifer in relation to the natural
ground surface were developed for 1978-1980 using ground water depth readings
from observation wells scattered throughout the project. Ground water table
maps of this nature were used to determine the potential effects of lowering the
water table on existing vegetation and general location of wetlands and wetland
habitats. These overlays also indicated the seasonal variation in water tables
and the general trend toward deeper water tables.

I

I

I

Readings from about 60 observation wells were used. The shallowest and greatest
readings, regardless of date, during a given year were plotted on overlays on
USGS 7.5-minute topographic bases. In general, the shallowest water table
depths occurred during late winter and spring before the growing season and
increased total evapotranspiration, while the greatest water table depths
occurred in the late summer and fall.

I

I

Using data supplied by the Bureau, the USGS conducted computer modeling analysis
of the Closed Basin to develop the well field systems to be installed for proj-
ect pumping. The analysis also provided estimated drawdown of the existing
unconfined aquifer in 2-foot increments within the project boundaries after more
than 50 years of pumping.

I

2. Effects.
8,460 of the existing
ect boundaries.

Construction and operation of the project will affect
43,122 acres of wetlands (wetland habitats) within proj-

I

I

An overall reduction of total plant (biomass) production may occur in wetlands
which are located over shallow water tables of 5 feet or less below the natural
ground surface. Also, the retention time of spring surface snowmelt runoff in
wetlands that normally receive annual runoff may be decreased. Lowering the
shallow water table beneath these wetlands could result in an increased soil

I

I

f

...._./1 /"\ _...",
( .l;J'J
~j . 4>--

I

I