FLOOD09853

• County: Denver
• Community: Denver
• Stream Name: South Platte River
• Basin: South Platte
• Title: Effects of the May 5-6, 1973 Storm in the Greater Denver Area
• Date: 12/31/1973
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

Effects of the May 5-6, 1973, Storm in the
Greater Denver Area, Colorado

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By Wallace R. Hansen

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ABSTRACT

Rain began falling on the Greater Denver area the
evening of Saturday. May 5, 1973, and continued
through most of Sunday. May 6. Below about 7,000 feet
altitude, the precipitation was mostly rain; above that
altitude, it was mostly snow. Although the rate of fall
was moderate, at least 4 inches of rain or as much as
4 feet of snow accumulated in some places. Sustained
precipitation falling at a moderate rate thoroughly sat-
urated the ground and by midday Sunday sent most of
the smaller streams into flood stage. The South Platte
River and its major tributaries began to flood by late
Sunday evening and early Monday morning.
Geologic and hydrologic processes activated by the
.May 5-6 storm caused extensive damage to lands and
to man made structures in the Greater Denver area.
Damage was generally most intense in areas where
man had modified the landscape - by channel constric-
tions, paving, stripping of vegetation and topsoil, and
oversteepening of hillslopes. Roads, bridges, culverts,
dams, canals, and the like were damaged or destroyed
by erosion and sedimentation. Stream banks and struc-
tures along them were scoured. Thousands of acres of
croplands, pasture, and developed urban lands were
coated with mud and sand. Flooding was intensified by
inadequate storm sewers, blocked drains, and obstructed
drainage courses. Saturation of hillslopes along the
Front Range caused rockfalls, landslides, and mudflows
as far west as Berthoud Pass. Greater attention to geo-
logic conditions in land-use planning, design, and con-
struction would minimize storm damage in the future.

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INTRODUCTION

The Greater Denver area has had a long his-
tory of intensive rainstorms and infrequent but
destructive floods, Though people are prone to
forget, memories were refreshed by the storm
of May 5-6, 1973, and the events that followed.
The weather forecast for Sunday, May 6,
called for cooler temperatures, considerable
cloudiness, and a chance of showers, Few peo-
ple, therefore, took much note of the onset of

a light drizzle late Saturday evening of May 5,
but by early Sunday morning, the magnitude of
the area-wide storm was becoming apparent to
all. By midday, many small streams were flood-
ing over their banks, reservoirs were spilling,
and canals were overflowing. The South Platte
River and its major tributaries were rising
ominously toward peak discharges late Sunday
night and early Monday morning.
Although the rate of precipitation on Sun-
day was not heavy by meteorological criteria,
the long duration of steady rain and the total
amounts in some places were exceptional. Some
backyard rain gages showed more than 4 inches
of moisture. The 24-hour depth measurements
at rainfall-runoff stations of the U.S. Geological
Survey ranged from 2.03 inches in Northglenn
to 4.38 inches in northeast Denver (Ducret and
Hansen, 1973, table 2), In northeast Denver a
100-year record was set for sustained precipita-
tion for a 24-hour period.
Above an altitude of about 7,000 feet, most
of the precipitation fell as snow, Twenty inches
of heavy wet snow fell over broad areas of the
mountains, and as much as 42 inches was re-
ported at Black Mountain, west of Conifer (The
Denver Post, May 9, 1973). Precipitation in the
form of snow retarded runoff in these areas
and, therefore, diminished the peak of flood dis-
charge. Conversely, infiltration of moisture into
the ground was increased, and, consequently, so
was the potential for landsliding. Had all the
moisture in the mountains fallen as rain, run-
off would have been more rapid, and flooding
would have been more intense, but infiltration
would have been less, and there would have
been fewer landslides.

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