FLOOD06420

• County: Costilla
• Stream Name: Rio Grande, Trinchera, Culebra, Costilla Creeks
• Basin: Rio Grande
• Title: Floodplain Information Report
• Date: 3/1/1989
• Prepared For: Costilla County
• Prepared By: CWCB
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.0 HYDROLOGIC ANALYSIS

3.1 Flood Characteristics

Flooding in Costilla County results from both heavy
snowmelt runoff and summer thunderstorms. Occasionally
snowmelt runoff is augmented by rainstorms. In the eastern
part of the county, which is more mountainous, snowmelt is the
predominant cause of flooding on the tributaries and the larger
subtributaries. On the smaller subtributaries thunderstorms
generally cause flooding. A review of the USGS Water Resources
Data reports for Colorado showed that most peak historic flows
had occurred in May. June, or early July. These reports do
not, however, include data for the smaller subtributaries. The
description of their flood characteristics is based on
discussions with county residents.

As was mentioned earlier, snowmelt-induced flows from the
mountains or flows from mountain thunderstorms are rarely
significant by the time the streams reach the western part of
the county. In that area floods are mostly caused by localized
rainstorms.

Snowmelt floods are characterized by sustained periods of
high flows, large total volumes, and marked diurnal
fluctuations. Rainstorm floods generally last only a few hours
with a rapid rise and a rapid fall in flows and small total
volumes. In southwestern and south-central Colorado rainfall
floods can result from general rain conditions caused by
Pacific storms in the late summer or early fall. Those floods
may be marked by sustained periods of moderately high base
flows with shorter periods of very high flows when intense
storms occur within the less intense general rains.

3.2 Historical Review

Although there is a record of significant flows at several
gages in Costilla County, historical accounts of flooding in
the county are very sparse. A review of newspaper from Pueblo,
Alamosa, and Costilla County revealed that there has been major
flooding on the Rio Grande and many of its tributaries in the
past. There were also accounts of major flooding in four
adjacent or nearby counties (Huerfano, Conejos, Alamosa. and
Rio Grande Counties). Although no detailed accounts of
flooding in Costilla County were found. there were brief
accounts of highway or railroad damage in and around the county.

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