FLOOD00880

• County: Mesa
• Community: Rifle
• Stream Name: Colorado River
• Title: Floodplain Information Report - Colorado River Mainstem From Rifle to Utah-Colorado State line
• Date: 4/1/1996
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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2.3 Basin Characteristics

The Colorado River has its origination upstream of Lake Granby in north central Colorado. The
headwaters originate in the Continental Divide to the north and east, in the Sawatch Range to the
southeast, in the Elk and West Elk Mountains to the south and in the northern and western slopes
of the San Juan Mountains.

Most drainages in western Colorado are either direct or indirect tributaries of the Colorado River.
This report, Volume 4, focuses on the Colorado River mainstem. The other three volumes of the
study address three major tributary streams. Volumes 1 and 2 provide information for the Yampa
River and the White River, which are tributaries of the Green River. The Green River joins the
Colorado River mainstem downstream of the study reach in Utah. Volume 3 provides
information for the Gunnison River, which joins the Colorado River at Grand Junction, within
the study reach. There are other western Colorado streams, not included in this study, which join
the Colorado River in Utah, downstream of the study reach for Volume 4.

The watershed area at the downstream limit of the study near the Utah stateline is approximately
17,843 square miles. Approximately half of that watershed area is in the Gunnison River
watershed. The elevation within the watershed ranges from more than 14,000 feet above Mean
Sea Level at the headwaters to 4330 feet at the stateline. The average slope within the study
reach is approximately 11 feet per mile, with a maximum of 16 feet per mile in the upper reach
and a minimum of 7 feet per mile near the stateline. .

The climate in the Colorado River basin is directly influenced by the varying topography. The
area west of Palisade to the stateline is classified as arid to semi-arid and is characterized by
moderate temperatures, low precipitation, low humidity, and frequent wind~ The higher mesa
areas are subject to more moderate to heavy precipitation. The mountains to the north, east and
south are subject to lower temperatures and greater precipitation. The primary sources of
moisture are air masses originating in the Pacific Ocean from October through April, and
airmasses from the Gulf of Mexico during the late spring and summer. Normal annual
precipitation is less than 10 inches in the Grand Valley near Grand Junction, from 10 to 20 inches
in the higher mesas, and more than 40 inches in the mountain regions. Occurrence of
precipitations is highly variable, with a large part of the total concentrated in a few months.
Much of the annual precipitation occurs as snow from October to April. Rainfall can occur as
convective-type thunderstorms during the late spring and summer months generally over the lower
elevations of the basin, making August, September and October normally the wettest months of
the year. The Colorado River basin, from its headwaters to the Utah stateline, is.shown in Figure
3.

2.4 Summary of Drainage Areas

The drainage areas for selected hydrologic points were determined through published reports,
planimetering, and established values at USGS gage locations. The results of the planimetering
were compared to the values in the published reports and at the USGS gages. The drainage areas
at the selected hydrologic points are shown in Table 2.

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