FLOOD10336 ‎(2)‎

• County: Adams
• Community: Brighton, Commerce City and Thornton
• Stream Name: South Platte River
• Title: Flood Hazard Area Delineation, South Platte River, Adams County
• Date: 4/1/2005
• Prepared For: UDFCD; Adams County; Brighton, Commerce City, Thornton, Metro Wastewater
• Prepared By: CDM
• Floodplain - Doc Type: Floodplain Report/Masterplan

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2-1

Surface water resources within the Study Area are influenced by upstream water supply reservoirs (Antero, Eleven
Mile Canyon, Cheesman Lake, and Strontia Springs Reservoirs), flood control reservoirs (Chatfield, Bear Creek,

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Below the confluence with Clear Creek and continuing to the downstream Study Area boundary, the land use
adjacent to the river shifts to predominantly reclaimed mine pits that are used as water storage and supply
augmentation reservoirs. From the confluence of Clear Creek to 88th A venue, these uses abut the channel, with
minimal riparian area. Residential subdivisions are located adjacent to the floodplain throughout the Study Area.
Suburban residential development extends to 88th A venue where land use changes to a mix of agriculture, gravel
mining/ water storage, and open space/ park land. Areas of relatively undisturbed riparian zones and abandoned
oxbows are encountered from Fulton Ditch to 168th A venue. Cottonwood stands, prairie dog colonies, and eagle
roost areas are present downstream of 88th A venue as existing urban development in the floodplain is minimal.
Residential, developments are found at the very downstream end of the Study Area,
east

commercial, and industrial
of the river in the City of Brighton.

In comparison to other river basins in Colorado, the South Platte River basin is relatively small. For example, the
Colorado River has a native annual supply of 6.74 million acre-feet (ac-ft) originating in Colorado, while the South
Platte River native supply is only 1.44 million ac-ft. However, more than 67 percent of the state's population
resides in the South Platte drainage basin. Although this concentration of population in the basin generates a large
demand for municipal water supplies, agriculture nonetheless represents the primary use of water in the basin. In
1994, withdrawals of surface water in the basin for agricultural use represented 69 percent of all withdrawals while
municipal uses represented 17 percent (McCarthy and Light 1995).

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In the upper reaches of the Study Area, from Franklin Street to Sand Creek, land uses in the floodplain are
primarily commercial and industrial. The MWRD's Central Treatment Plant (CTP) and the Xcel Energy
Corporation's (Xcel) Cherokee Power Plant are located in this reach together with several industrial parks. The
Denver Water reuse plant, under construction at the time of this study, on the east overbank upstream of the York
Street river crossing, adds to the industrial presence in this area. At the time of this publication, active sand and
gravel mining is occurring at several locations in the Study Area.

2.4 Surface Water Resources

Post-Piney Creek Alluvium. These deposits consisting of sands, gravels, and cobbles are found within the
modern floodplains of the South Platte River and the alluvial materials along the tributary creeks.

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Younger Eolian Sand and Silt. Extensive deposits of wind blown
Platte River valley.

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sand and silt are found southeast of the South

· Piney Creek Alluvium. Deposits of humic sand, silt, and clay with some smaller gravels form
straths along both sides of the South Platte River valley.

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Adams County along the South Platte River is a spectrum of urban and rural areas, with land uses near the
upstream study limit being the most urbanized and the corridor becoming progressively more rural downstream.
At the downstream boundary, the Study Area is a mix of agriculture and sand and gravel mining.

terraces and

Broadway-Louviers Alluvium. Within the Brighton quadrangle a single large terrace, in places over 1
wide, lies 30 to 35 feet above the river along the east side of the South Platte River valley.

mile

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2.2 Land Use

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Older Eolian Sand and Silt. The upland surfaces west of the South Platte River valley are extensively mantled
by Eolian fine sand and silt and some clay deposits

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Lying in the rain shadow of the Central Rocky Mountains, the Study Area frequently experiences periods of low
precipitation, with an average annual precipitation of 13 to 17 inches. Runoff sources in the watershed include
precipitation, occurring primarily during summer thunderstorms from April to September, and snowmelt from the
mountains in the upper watershed. Snowmelt generally begins in late spring, and can continue for several months,
depending upon the depth of snow pack and weather conditions. Temperatures in the Study Area average 45 to
50 degrees F annually.

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· Slocum Gravels. Thin deposits of gravels mixed with fine grained materials are occasionally exposed at
elevations below the Verdos surface west of the South Platte River valley. The deposits are generally between 2
to 4 feet thick.

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The Study Area for this report is bounded on the south (upstream) by the Denver/Adams County line at Franklin
Street and on the north (downstream) by the Adams/Weld County line at 168th Avenue. On the east and west
sides, the Study Area is generally contained between Riverdale Road and Brighton Road, respectively. The South
Platte River channel in the Study Area slopes gently northeastward with a gradient of 9.1 feet/ mile or 0.17 percent.

Verdos Gravels. The Verdos gravels are the oldest surficial deposits. The Verdos deposits occupy the
uppermost surfaces on a line of hills lying west of the South Platte River valley, and are also found near Barr
Lake.

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2.1 Geography and Climate

The headwaters of the South Platte River watershed lie along the Continental
Leadville, Colorado. From the mountains, the river flows north/ northeast for 450 miles to its c
North Platte River in North Platte, Nebraska, where they join to form the Platte River, which then flows
Missouri River. The total South Platte River watershed extends over an area of 23,000 square miles

Divide in the Rocky Mountains near
onfluence with the
into the

Incision of the current South Platte River Valley occurred during the Pleistocene Epoch. The valley floor alluvium
is mostly reworked gravel overlain by a few feet of rich sandy silt. Underlying the alluvium is bedrock of the
Arapahoe Formation of Cretaceous Age and the Denver Formation of Tertiary and Cretaceous Age (Robson 1996).
Generally, the alluvium exists beneath and in the riparian zones of the river. However, in some isolated places, the
river is in direct contact with the bedrock. The surficial deposits found in the South Platte River valley in Adams
County include the following (UDFCD 1985a):

2.3 Geology

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tudy Area Description

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ection 2

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4/8/05