FLOOD01761

• Designation Number: 83
• County: Larimer
• Community: Unincorporated Larimer County
• Stream Name: Big Thompson River
• Title: Floodplain Information Report - Big Thompson River, Loveland to the Larimer-Weld County Line, CO, and the Little Thompson River, Boulder and Larimer Counties, CO
• Date: 6/1/1977
• Designation Date: 11/1/1977
• Floodplain - Doc Type: Floodplain Report/Masterplan

T<lble , OataY
Big Thompson R1ver BasIn Vicinity Cllmatologklll
'1eanAnnuIII July Mean J<lnullryMean
Location Procipltation Temperature Temperature
(Ioches) (OF) (OF)
Fort Collins 14.94 70.8 26.8
2 miles East-
Southeast of
Post Office '"
Longrnont 12.74 71.6 26.8
Greeley 12.20 73.3 24.5
Estes Park 15.87 62.2 27.1

The Little Thompson RIver, a right-bank tributary of
the Big Thompson RIver, draios an area of about 200 sqU<lre miles.
The rtver has its source at the eastern edge of Rocky Mountatn
NatIonal Park near T~in Sisters Peaks <lnd flows easterly to join
the Big Thompson River about 1 mile north of Mi Illken. Eleva-
tIons In the basin range from about 11.000 feet "'.s.l. at the
head~ators to about 4725 feet m.s.l. at the 81g Thompson River
confluence.

!J

Thi 5 d<l-ta ~as compi led from National Weather Service reoords
for 1941-1970.

The reach of the Lit-tle Thompson River studied tn this
report begins approximately on the section line between Sections
2 and 3, T3N, R70W and ends about 10 mIles downstream II-t the
larimer-Weld County lIne between SectIon 24, T4N, R69W and Sec-
tionI9,T4N,R68w. The L1ttleThompson River throughout this
reach Is characterIzed by a meanderIng channel flowIng through a
"v"-shapedvalley. Channel dimensions vary through the study
reach. Fromrefereocepoints I t046,thechannel width varies
from 60 to 160 feet and averages 100 feet; the depth ranges from
7 to 9 feet. From reference points 46 to 79. the channel width
varies from eo to 140 feet and averages 90 feet; the depth ranges
from4t09feet. Frorn reference points 79 to 104,theehannel
width varIes from 60 to 130 feet and averages 90 feet; the depth
ranges from 4 to e feet. The streambed has an average slope of
17 feet per mile from reference points I to 46, 2~ feet per mile
from reference points 46 to 79, and 17 feat per mi Ie from refer-
ence points 79 to 104.

The prim<lry stre<lms In the Big Thompson RIver basin are
the Big Thompson River <lnd the LIttle Thompson River. The Big
Tnompson River has Its source In Rbcky Mountain National Park near
1>\1 Iner P"ss. From -thIs are" the river flows in a gen!trlll easterly
direction to Its confluence with the South Platte River about 5
mile,> ~outh..",,;t of r,r"..I"y. The ~ortlon of th.. 81g Thorn!',,;on River
studied In thIs report begins about 2 mIles southeast of Loveland
ne<lrthecenterofSection20,T5N,R68Wllndendsllbout5mlles
downstream <It the L<lrimer-Weld County lIne botween Soction 24,
T5N, R68Wand Soctlon 19, T5N, R67W. Thaupstreamstudy limltof
this report is the do~nstream study limit of FlOOd PlaIn Informa.
tlon. BI~ Thompson RIver. Love I and, Colorado, pub I ished by the
Corps of [ngioeers In December 1911. Througnout this re;')Ch tne
channel wIdth yarles from
..Idth. The channel depth
feet. The stronm~d has
through the study reach.
through a ",,"-shaped valley SC<'lrred by considerable mining IIctivify.

eo '0 '60 foet and averages 100 leet '"
ranges from 4 '00 feet and averages ,
an <lver<lge Slape of o.s feet per mile
The eMnnel Is alluvial '"' meanders

F;gur,,~ I lhrough ~ sno~ existing condition:; "Ion~ tho
Big Thompson River study reach. Exlstlngconditionsalonofho
Little Thompson River study reaCh are sho~n on figures 5 through
I I. These photographs were taken In December 1976 by personnel
of the Colorado Water Conservation Bo~rd.

.

,