C150221 report ‎(2)‎

• Contract/PO #: C150221
• Contractor Name: San Luis Valley Irrigation District
• Contract Type: Grant
• Water District: 0
• Loan Projects - Doc Type: Report

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downstream of the dam the river valley is highly entrenched for appro:cimately 0.25 miles before opening
up into a wider floodplain.
The study area is was defined by the proposed high-water elevations of the two dain proposals. The first
dam proposal would increase the height of the eacisting dam while the second proposal includes the
construction of a new dam to be located approximately 1.4 miles downstream of the existing dam.
WETLAND ASSESSMENT METHODOLOGY
Wetlands and other waters of the U.S. (WOUS) in the study area were identified on September 14`� and
' 20t`', 2006, using the methodology defined in the Routine Determination procedure set forth in the U.S.
Army Corps of Engineers Wetlands Delineation Manual (USACE 1987). Wetland boundaries were
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defined based on presence of hydroph}rtic vegetation, hydric soils, and hydrologic indicators that under
normal conditions would indicate wetland conditions. Where wetland conditions did not occur adjacent
to surface water, the jurisdictional boundary was identified based on evidence of the ordinary high water
mark (OHWM) (USACE 2005). Both ttpsh•eam and downsheam wetlands have been altered as a result of
the regulated stream flows and prolonged periods of inundation. Therefore any areas located below the
' OHWM of the reservoir were considered to be atypical areas, as defiiied by the '87 manual. Sugnet
generally delineated the OHWM based on the observed physical indications such as a lack of vegetation,
water stains, rack lines, and historic aerial photos and mapping. Sugnet did not delineate wetland areas
below the OHWM of the reservoir. The primary areas of concern are those areas located above the
e:cisting OHWM and below the proposed OHWM, which was conservatively estimated in the field based
on the maps provided by CDM. Wetland axeas located below the dam were also delineated as atypical
wetlands due to the fact that the dam has altered natural stream cycles. The regulated flow will reduce the
potential for high water events, which lowers the frequency of flooding. The altered hydrologic regime
may induce a slow change in the vegetative communities of the floodplain, and also removes natural
stream channel and floodplain morphological processes. These considerations were taken into account
during the delineation of the floodplain areas located below the dam. Therefore the delineation is
conservative estiination of the actual wetland boundary.
Sections of the wetland boundaries as well as point transects were survey-located using Trimble ProXR
GPS (sub-meter accuracy) and are depicted on Figure 3 within Attachment A. The sections and transects
were overlaid and geo-referenced to aerial photographs of the site. The GPS boundaries and transects
555 Rivergate Lane, Bt-101 Durango, Colorado 8�307 Ph.970.259.9595 Fax.[phone] www.sugnet.com