FLOOD07275

• County: Costilla
• Community: Costilla County
• Stream Name: Trinchera Creek, Costilla Creek
• Basin: Rio Grande
• Title: Floodplain Information Report
• Date: 3/1/1994
• Prepared For: Costilla County
• Prepared By: CWCB
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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,'igure 4, Map of e1ajor Reservoirs

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3.6 Peak Flows

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The final task remainin':l in the hydrologic analysis was to
consider all of the factors discussed above and synthesize them.
A discharge profile for each of the three major tributaries, from
th., furthest r,jdge upstream all the way to the confluence with the
Rio Grande. was needed. Because Of the pattern of stream gages in
the county. the most difficult part Of this task. was the
determination of flows from the middle of the county west
(downstream) to the Rio Grande. For a very short time (less than
one year) there were stream gages at the mouth of each Of the
three major tributary streams. These gages were not maintained
long enough to provide meaningful data. The gages that do provide
such long, term data are mostly clustered in and near the Sangre
de Cristo foothills.

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Determining discharge prOfiles was difficult for other reasons
as well. Downstream Of foothills locations like f'ort Garland, San
Luis. and Garcia average fLows are significantly reduced. Factors
contributing to these reductions in average annual flows as one
travels from east to west are valley storaqe. greater losses due
to infiltration into sandier soils. reservoir storage. irrigation
diversions from streams that are some of the most overappropriated
in Colorado, inadvertent detention by highways. roads, and
agricultural activity, and significantly reduced precipitation.
The area around Alamosa, the eastern part of Conejos County. and
the western part of Costilla County has the lowest average annual
precipitation of any area in Colorado accordinq to the National
Weather Service.

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As a result of these factors. the flOOdplains in the
downstream portion Of each of the three major basins are not as
obvious in the field as the floodplains farther upstream. A
detailed calculation Of that part of each stream's discharge
profile would require reviewing reservoir records and irrigation
records and performing hydrologic routings to determine losses due
to valley storage and inadvertent detention by highways, roads,
agricultural features and so on. Such an analysis was beyond the
scope of this study. For that reason, it was decided that the
peak flow determined at the qage that was the farthest downstream
on each of the three major streams should be used from the
location Of that gage downstream to the stream's confluence with
the Rio r.rande. In view of the above discussion about flow
dttenuation. that is a vllry conservative assumption.

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The and lysis conducted for TM-l and for the SCS study of Rito
Seco were borh plotted on one graph, showing drainaqe area on the
independent axis and the ratio of the calCUlated peak 100-y~ar
flow to drainage area (cubic feet per 6econd per ~quare mile)

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