FLOOD06098

• Designation Number: 84
• County: Adams
• Community: Unincorporated Adams County
• Stream Name: South Platte River
• Basin: South Platte
• Title: Flood Hazard Area Delineation - South Platte River, Adams County, CO
• Date: 9/1/1977
• Designation Date: 11/1/1977
• Floodplain - Doc Type: Floodplain Report/Masterplan

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FLOOD HISTORY

In 1965, a unique combination of orographic effects and meteo-
rological conditions in the South Platte River Basin caused the
worst flooding in the region's recorded history. Severe thunder-
storms commenced over the headwaters of Plum Creek and Cherry
Creek on June 16, and moved northeasterly down the creeks fol-
lowing and augmenting peak flows. More than 14 inches of rain
was recorded at Palmer Lake in 4 hours. Overnight, westerly
winds moved the storm front to a position over the Kiowa and
Bijou Creek Basins where it met with thunderstorms forming just
south of Agate. Here 5.25 inches fell in 45 minutes. The net
results of these conditions were six persons drowned, two other
deaths caused by flood-related activities, and estimated damages
of $500 million in the South Platte River Basin, of which $300
million occurred in the Denver area. (Ref. 4)

Major floods in the Adams County area have been observed and
recorded on the South Platte River and its tributaries since
1844. During that period, eleven major floods have occurred.

In 1844 and 1864, reports stated that "bottom1ands near Den-
ver were covered with water bluff to bluff" (Ref. 5). By
1876, encroachment into the flood plain had developed to
such an extent that on May 23, 1876, the Rocky Mountain News
reported that, "(The South Platte River) was higher to be
sure ... several feet. higher perhaps in 1864 ... but it was
not able to work such destruction at that time as now. There
was not so much town here in 1864, as now, nor as many bridges."
(Ref. 1).

Floods on the South Platte River have been generated near the
slopes of Monument Divide, a high ridge located between Castle
Rock and Colorado Springs, extending from the Rocky Mountains
down to the plains near Limon, Colorado.

In 1973, the May snowpack over the South Platte River tributary
basins varied from normal to 170 percent above normal. Early
warmer temperatures at higher elevations caused abnormally high
snowmelt runoff, which was augmented by the prolonged rainfall
of May 5 - 6 to produce the worst flooding on the South Platte
River since June 1965. Urban, rural and transportation flood
damages were estimated at $99,500,000 (Ref. 3). Of these, the
rural - agricultural category registered the greatest amount of
damages, because of the extensive damage to public and private
irrigation works and the widespread loss of crops due to inunda-
tion, erosion, and sedimentation. Massive deposition of sediment
on agricultural lands made it impossible to produce crops for two
years. High water marks observed during this flood are shown on
the drawings and recorded in the F100dway tables in the appendix.

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The most significant floods of recent times on the South Platte
River occurred in 1912, 1921, 1933, 1935, 1942, 1965, and 1973.
The discharges for these floods were 13,000 cubic feet per
second (cfs), 8,790 cfs, 22,000 cfs, 12,320 cfs, 10,200 cfs,
40,300 cfs, and 33,000 cfs, respectively, at the Henderson
gage (located near Henderson, Colorado).

Past floods have resulted from snowmelt and intensive rain
storms over the mountain tributaries, rainstorms over the
eastern tributaries, and combinations of these conditions.

Scenes of historic floods in Adams County are shown in Figures 1
through 8.