FLOOD10167

• Designation Number: 299
• County: Costilla
• Community: Unincorporated Costilla County
• Basin: Rio Grande
• Title: Floodplain Information Report - for the Rio Grande, Trinchera Creek, Culebra Creek, Colstilla Creek, and Selected Tributaries in Costilla County
• Date: 3/1/1989
• Designation Date: 11/1/1989
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

3.3 Stream Gaqe Data

A hydrologic analysis was performed to determine 100-year
peak flows on streams in Costilla County. The principal data
sources were the USGS publication. Technical Manual No.1.
Manual for Estimatinq Flood Characteristics of Natural-Flow
Streams in Colorado (TM-l). published in 1976. and a USDA Soil
Conservation Service (SCS) hydrologic analysis for the Town of
San Luis. prepared as a part of a 1979 flood control project
study that was conducted by the SCS. These two sources were.
in turn. principally based on analyses of gage records for
sites in Costilla county.

TM-l included gage analyses for five (5) stream gages in
Costilla County. These gages are listed in Table 4. The SCS
analyzed five (5) gages also. Three of those gages had already
been analyzed in TM-l. The results of their analyses are
listed in Table 5.

Since it had not been analyzed for TM-l or the SCS study.
one more gage was studied by the CWCB. The gage on Trinchera
Creek below Smith Reservoir was felt to be important because it
had over 50 years of record and because it provided data
further downstream on any of the three major tributaries than
any other gage in the county. Even though it is located below
a reservoir. the gage provided helpful data.

The site is actually downstream of two reservoirs. Mountain
Home Reservoir is upstream of Smith Reservoir on the mainstem
of Trinchera Creek (see Figure 4). Sangre de Cristo Creek does
not flow through Mountain Home Reservoir. but joins Trinchera
Creek at Smith Reservoir. farther downstream. Sangre de Cristo
Creek actually contributes a much larger drainage area to the
Trinchera Creek basin above Smith Reservoir than does the
Trinchera Creek mainstem.

The question of what role either of these reservoirs might
play in flood flows on Trichera Creek can. however. be put
aside. As is discussed in Section 3.4 (Reservoirs). ultimately I
no flood storage credit was given to any of the reservoirs in
Costilla County for the purposes of this study. 61 square miles
on the Trinchera Creek mainstem are tributary to Mountain Home
Reservoir. These were not subtracted from the total drainage
area at the gage below Smith Reservoir (396 square miles). The
calculated 100-year flow of 1.560 c.f.s.. based on the
assumptions described in Section 3.4. provided useful
information for the discharge prOfile for Trinchera Creek west
of the foothills.

Additionally one gage that had been analyzed for TM-l. the
gage on Costilla Creek above Costilla Darn in New Mexico. was
restudied by the CWCB. For reasons that are unclear. several
years of record on that stream. inCluding an exceptionally
large flow in 1954. were not made part of the TM-1 analysis.

-20-