FLOOD10332

• County: Bent
• County: Prowers
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Arkansas River from John Martin Dam to the Colorado/Kansas State Line: Channel Capacity and Riparian Habitat Planning Study
• Date: 7/1/1999
• Prepared For: CWCB
• Prepared By: USACE
• Floodplain - Doc Type: Floodplain Report/Masterplan

2.2 CLIMATE

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The climate in Prowers County is characterized as semi-arid/continental with low and
variable precipitation, low hwnidity, and a wide seasonal range in temperature. Weather
patterns generally are governed by dry air from the southwest;. however, winter storms emanate
from the northwest, and moist air from the Gulf of Mexico frequently influences weather
during spring, summer, and fall.

During December through February, nighttime temperatures are usually below 32 OF
while daytime temperatures generally are above freezing. In June through August, the daily
maximum temperature is 90 OF or higher on about 70% of the days. The length of the freeze-
free growing season in Prowers County is approximately 166 days (pannell et aI. 1966).

Average annual precipitation at Lamar is about 15 inches, nearly 60% of which result
from thunderstorms in May through August. The annual snowfall is about 24 inches (pannell
et aI. 1966). Because rainfall is low and summer teIp.peratures are high, evaporation rates are
high during the growing season. Average annual pan (Class A) evaporation at John Martin
Dam is 84 inches, of which approximately 66 inches occurs during April through October
(USACE 1983). The months of November through March account for approximately 20% of
the annual pan evaporation.

2.3 PHYSIOGRAPHY, GEOLOGY, AND SOILS

The Arkansas River in Prowers County lies within the High Plains section of the Great
Plains physiographic province (Fenneman 1931) and is characterized by flat to gently rolling
uplands with a few shallow valleys and many shallow, undrained depressions. The alluvial
bottomland along the river ranges from one to three miles wide and is bounded on the north
and south by terraces or sand hills. Slopes within the valley bottomland are less than 2%.

Bedrock underlying the Arkansas River consists of Cretaceous sandstone, shales, and
limestones. Most of the channel is within the Lower Cretaceous Dakota formation, composed
of fine-grained sandstone and sandy shale. Saturated valley-fill alluvium consisting of gravel,
sand, silt, and clay of Pleistocene to Holocene age and 50 to 150 feet thick occupies a trough
eroded in the sedimentary bedrock.

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Soils within the Arkansas River floodplain belong to the Las-Glendive association _
which consist of alluvial materials that vary extremely in texture, depth, and drainage. These
soils include sand and gravel, silt loams, and clay loarns (pannell et aI. 1966).

2.4 WATER QUALITY

The Arkansas River comprises Colorado's largest drainage basin. Originating in the
Rocky Mountains in the central portion of the State, the river flows eastward for about 235
miles before entering Kansas. The drainage area between John Martin Dam and Coolidge,