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5.7 Biological Resources

species composition within the grasslands based on soil type, grazing pressure, slope and
aspect, available groundwater, and disturbance history. As is typical in California, both native
and non-native species occur within the mosaic of grassland types. Wildflower fields are a
component within portions of the grasslands and occur throughout most of the Project’s on-
site and off-site study areas.

Riparian and wetland vegetation types occur in association with the drainages, springs, and
seeps located on the Project site. Oak woodland vegetation types are dominant in the
western portion of the Project site and in the southern portion of the Project site mainly in
the areas south of SR-138. These consist of dense stands of both deciduous and evergreen
oak species, such as blue oak (Quercus douglasii) and Tucker’s oak (Quercus john-tuckeri).
Scrub vegetation types (primarily chaparral) are generally found in the western portion of
the Project site on somewhat eroded, steep slopes. Rabbitbrush scrub is present on the lower
elevations of the Project site. Vegetation types are described further below, and a list of
dominant species in each vegetation type is also included in Appendix 5.7-A.

Geology and Topography

The topography of the Project site ranges from steep to flat but, in general, the site is
characterized by gently rolling hills. Steeper hillsides that gradually decrease to more
moderate slopes occur to the west, while flat topography occurs on the easternmost portion
of the site. Elevations range from approximately 3,635 feet above mean sea level (msl) along
a ridge overlooking Quail Lake to approximately 2,975 feet above msl in the alluvial drainage
area in the east portion of the site.

Erosion is a major element of many land features and is seen throughout the site, particularly
on steeper slopes where vegetation is lacking and deep eroded gullies have formed.
Meandering watercourses have also created numerous gullies ranging in depth from a few
inches to over ten feet.

Several major topographic and other physical features are located on the Project site. Two
primary drainages are located on site. Tentrock Creek is an east-west trending drainage that
parallels SR-138. The other is Oso Creek, which enters the site from the northwest and
meanders southeast and east in the northern portion of the site.

A series of high ridges in the Project site separate a number of intermittent or ephemeral
streams that have high flows following winter storm events.

The California Aqueduct splits into the West and East Branches just north of the Project site.
The Aqueduct’s East Branch is located just north of and adjacent to the northeastern segment
of the Centennial boundary. The West Branch bisects the Project site into an eastern portion
and a western portion. The West Branch leads to Quail Lake, which is a former natural pond
on the San Andreas Fault that has been reconfigured into a reservoir as part of the Aqueduct
system. Quail Lake is situated adjacent to the southwest portion of the Project site. Although
several streams cross under the Aqueduct via culverts, the Aqueduct itself does not connect
to any on-site drainages.

Tentrock Canyon Creek, a nearly perennial stream that flows in a northerly direction, crosses
SR-138 near the southeastern corner of the Project site and flows into the lower portion of

R:\Projects\PAS\CEN\000306\Draft EIR\5.7 Bio_051117.docx 5.7-11 Centennial Project
Draft EIR