FLOOD04555

• County: Dolores
• County: Montezuma
• Stream Name: Dolores River
• Basin: San Juan/Dolores
• Title: Flood Hazard Information
• Date: 9/1/1978
• Prepared For: Dolores, Montezuma
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

he Dolores River is tributary to the Colo-
ra River, joining that stream in eastern Utah
ab ut 100 airline miles northwesterly of
Do ores, The West Dolores River joins the
Do ores River from the north aboul 15 miles
up ream from Dolores. Both streams rise in the
Sa Juan Range, which reaches elevations
ar nd 14,000 feet. Much of the higher
dr nage area is timbered with pine, fir, and
spr ce while low brush dominates in the lower
ele ations around Dolores. Lost Canyon Creek,
wh ch joins the Dolores River from the south at
Do ores, rises at elevations of around 10,000 feet
on he western slopes of the La Plata Mountains,

N lURE OF FLOOD PROBLEMS

eneral frontal type rainstorms approaching
fr the southwest can occu r over the Dolores
Ri r Basin from mid-June through December,
bu records show that they occur most often
du ing September and October. Convective
ty cloudburst storms occur frequently in
so thwestern Colorado in the summer. Much
of he annual precipitation in this region occurs
as now and a deep snowpack usually accumu-
lat s in the high elevations, Consequently,
sn wmelt runoff in spring and early summer
co stitutes a frequent but comparatively mod-
er e flood threat because high peak flows are
no characteristic of snowmelt runoff. Cloud-
bu st storms I'roduce high intensity rainfall but,
du to the small areal extent of this type of
st m, their short duration, and small volume of
ru off, constitute a significant flood threat only
the smaller drainage basins. Cloudburst
ms constitute the most severe flood threat
g Lost Canyon Creek. In general, however,
tal type rainstorms constitute the most

F lURE FLOODS

Although floods with magn itudes and fre-
q ncies similar to those of floods that have
o urred in the past could recur in the future,
di ussion of future floods in this report is
Ii ited primarily to one designated as the 100- .
ye r flood, A lOa-year flood has a peak flow
m gnitude with a 1 percent chance of being
e ailed or exceeded in any given year, and a
fr uency of occurrence of about once in 100
ye rs on the long-term average, It must be
u erstood that the term "lOa-year flood"

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Characteristically, terrain in the Lost Canyon
Creek drainage area is undulating with stands of
pine and fir above 9000 feet and sagebrush,
pinon pine, and juniper over most of the
tributary drainage,

In total, the Dolores River drains about 570
square miles upstream from Dolores. This part
of the drainage basin is situated in southwestern
Colorado. Elevations range from about 6900 feet
at Dolores to over 14,000 feet in the high
headwater regions, Stream gradients upstream
from Dolores average about 30 feet per mile on
the Dolores and West Dolores Rivers and 110
feet per mile on Lost Canyon Creek,

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serious flood hazard in the study area. The
largest flood of record, which occurred in
October 1911, resulted from this type of storm
on the drainage areas of the Dolores and West
Dolores Rivers. Much of the town of Dolores
was flooded at that time. Heavy rain caused
flooding along Lost Canyon Creek in October
1941. In May 1973, snowmelt runoff damaged
areas along the Dolores River reach studied for
this report and threatened other riverine areas.
A bridge was washed out on Lost Canyon
Creek. Large snowmelt flows also occurred on
the Dolores River in May 1941 and June 1949,
The largest recent flood on the Dolores River
occurred in September 1970, but information
other than magnitude of flow for that flood is
not available, Flood losses in the study area
usually consist of damage to bridges and
irrigation structures, In a number of flood years,
flood fighting was required to protect residen-
tial and commercial areas from rising water.

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relates to flood magnitude and does not mean
that the flood will occur one time in a lOa-year
period, Essentially, "probability of occurrence"
is implied. Thus, during the term of an average
mortgage (30 years), the chance of a lOa-year
flood occurring is approximately 1 in 4; during
an average lifetime (70 years), the chance is
approximately 1 in 2. In a lOa-year period, the
chance of a lOa-year flood is approximately 3 in
5.

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