FLOOD09948

• County: Hinsdale
• Community: Lake City
• Stream Name: Gunnison River, Henson Creek
• Basin: Gunnison
• Title: Floodplain Information Report
• Date: 10/1/1985
• Prepared For: Lake City, Hinsdale County
• Prepared By: Westwater Engineering
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

Cralnage BaSins and CharacterlSt!CS
;he jpper 1rainage basi1s of Henson Creek and the Lake Fork Of the
Gunnison are similar In area ana physical characteristics. Henson
Creek has a drdlnage area of 'll.a S!luaremiles. The Lake Fort of tne
GunnIson has a drainage area of 146.8 sQuar~ miles above the mouth of
Henson Creek. Henson Creek flows primarily in an easteriy direction with
tributaries coming in from the ~rth and south. The Lake Fork of the
Gunnison initially flows easterly in the upper half anc in the lower half
it takes a long, 90Q bend northward before entering Lake City.
The following information on the Lake Fork of the Gunnison was
taken from reference ~2.
Mean8asinElevation-l1,500ft. Land Use:

ChannelSlope-.396
Channel Length-23.7mi.
Basin Length - 13.8 mi.
Basinlolidth-8.3mi.
Channel Aspect-740
Basin Aspect 550
Henson Creek has a similar elevation, topography and land use.

Forest - 66%
Agricultur€' 0
Rang€' 28%
Water 0
Urban 1'1;

T~e San Juan Mountair.s in the study area are comprised primarily of
granitic and volcaniC rocks. Numerous rock outcrops are present. The
valley floors are compri sed of allu~ I al and co 11 U'l i a: cepos i ts or roCkS
ftem these sources. A thin layer of loam overlies the valley floor
deposits in some areas.
Characteristics of Different Reaches
The study reaches for Lake For~ of the Gunnison River and Henson Creek are
shown on Plate la. The lower reach of the Lake Fork of the
Gunnison River extends downstream approximately 2 miles from the confluence with
Henson Creek through the San Juan Ranch resort. The upper reach of detailed
study of the Lake Fork of the Gunnison River extends upstream approximately
D.7milesfromtheconfluence'ilithHensoncreektoabout700feet below Crooks
Falls. Approximate 100-year flood elevations have been estimated for the
700 foot stretch of river between the last upstream cross-section and Crooks
Falls. TheHensonCreekreachextendsaboutO.33milesupstreal1lfromitsmouth
through the city limits. The Lake Fork was divided into two separate study reaChes
since peak flows used in analysts are different above and below the confluence
with Henson Creek.
The stream valley of the Lake Fork within the study reaches is generally
characterized by a relatively flat flood plain. 600 to 800 feet wide, situated
between steep, heavily wooded mountain slopes which rise to over 10,000 feet
~n either side. The flood plain consists of allu~ial deposits of boulders.
cobbles and sand with an overlying layer of topsoil. Occassional outcrops of
bedrOCk can be found along the stream channel. Vertical bluffs - 20 to 60 feet
high _ oorderthe Lake Fork on the northeast part of Town. T~ flood plain is
subject to erosion and extreme changes tn the stream channels from floods.
Henson Creek emerges from the mountains through a narrow gorge onto the
Lake City flood plain. 80th streams flow [n well-defined channels within the
flooo plaln. Trle Lake Fork of the Gunnl>on lIa, " t.:;nil;;ncy to "1;;Jn;:::~r. whereJs
Henson Creek tends to be more confined to its existing channel (see Plates 16
through 24 for cross-sections of channels of both streams). O~er the years,
the adjacent banks of both channels have been built up by the TOwn and County
in an atte~pt to control the location and reduce erosIon potential from flood
flows. Howe~,"r, future floods wi 11 conti nu€ to exert pressure to shi ft stream
channels dS they ha~e in the zast.

Elevations in the study area range from over 14,000 feet in the
western headwat~rs to 8670 feet at Lake City. Topography. particularly
In the side basins and hNdwaters, is very rugged ~nd :;teep. TritlutJr:es
may drop 3,000 to 4,COO feet over 3 to 4 miles. The main ~alleys of the
Lake Fork dnd Henson Creek have a more gentle gradient belOW 10,000 feet.
Three types of ~egetation zones are present in the study area: AlpIne,
Sub~alpine and Montane. Grasses and sedges grow in the tundra zone
above tree line. The tr~es are compris€'d primarily of spruce, fir and
aspen,withsomecottonwoods in the valley floor. In the lower areas,
there are some irrigated pastures and sagebruSh meadows.
Approximately 300 ac~e feet of irrigatIon water are di~erted annually
on the Lake Fork above Lake City. The Lake Fork also flows through Lake
San Cristobal. a natural lake created by a large mud slIde in the ~arly
1900s. '~hICh Jttenuates its flows. There are no <Ii versions or dams of
significance on Hensen Creek.

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