FLOOD02581

• Designation Number: 557
• County: Gunnison
• Community: Unincorporated Gunnison County
• Basin: Gunnison
• Title: Hydrology Addendum
• Date: 2/1/2002
• Prepared For: Gunnison County and CWCB
• Prepared By: Montgomery Watson Harza
• Floodplain - Doc Type: Floodplain Report/Masterplan

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STUDY AREA DESCRIPTION

Community Descrivtion
Gunnison County is located in western Colorado. The county is bordered by Mesa and Pitkin
Counties on the north, Delta, Montrose and Ouray Counties on the west, Hinsdale and Saguache
Counties on the south and Chafee County on the east. The average altitude of the Gunnison
River Basin above the study area is relatively high, ranging in elevation from approximately
7,500 feet above mean sea level as the Gunnison Ri ver leaves the county to over 14,000 feet at
the high peaks of the Continental Divide, Gunnison is the county seat of Gunnison County. The
Colorado Division of Local Government Demography Section estimated the population of
Gunnison County as 13,956 in April of 2000. The area covered by the PIS includes only the
unincorporated areas of Gunnison County,

Watershed Descrivtion
The Gunnison River, formed by the confluence of the Taylor and East Rivers at Almont, has a
well-developed stream pattern with inflowing tributaries primarily from the west and north, The
larger of these tributaries contributing to the flooding problems in the study area include Ohio
and Tomichi Creeks, A majority of the watershed is undeveloped, However, the communities of
Gunnison, Crested Butte and Mount Crested Butte are developed within the watershed, In
addition, Taylor Park Reservoir, located on the Taylor River prior to its confluence with the
Gunnison River, affects a portion of the flood flows in the Gunnison River. Blue Mesa
Reservoir, located on the Gunnison River downstream of Gunnison, affects flood flows on the
river downstream of the reservoir. However, there are no designated approximate floodplains
downstream of the reservoir.

The North Fork Gunnison River is formed by the confluence of Muddy and Anthracite Creeks
near Paonia Dam. From its source to its mouth near Lazear, the stream flows approximately 30
miles on a southwesterly course, The watershed is primarily undeveloped. Paonia Dam has the
potential to affect flood flows on the North Fork Gunnison River,

There is a small portion of the Crystal River within the study area, The Crystal River is tributary
to the Roaring Fork River near Carbondale, The community of Marble is located on the Crystal
River, although the small amount of development likely does not have a significant impact on
flood flows,

Floodin!!: Problems and Floodin!!: History
While slopes are steeper in the upper drainage basins, the dense vegetation in these areas serves
to reduce rainfall runoff. A greater percentage of runoff is permitted by the sparse vegetation of
the foothills and pastureland at the lower elevations. Consequently, the majority of the flood
problems in the study area are caused by rapid snowmelt, This type of flooding has a long-
duration runoff, which results in a continuous rise in water leveL

Flood potential also exists from long- and short-duration rainstorms, Floods from short-duration
rainstorms are characterized by high peaks and small volumes. The potential for the most severe
flooding is created by the combination of rapid snowmelt with long-duration rainfalL

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