FLOOD11751

• County: Statewide
• Basin: Statewide
• Title: Floods in Colorado
• Date: 1/1/1948
• Floodplain - Doc Type: Educational/Technical/Reference Information

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sm.raction of

E\ CHAX::\EL CAPAClTY

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of melting snow, and even then the flow is only a
what it was before di versions for irrigation ,vere made This reduc-
tion in flow has resulted in a gradual choking of the channels by
sediment and vegetation, until eventual.ly their capacity has become
so small that when floods occur the overflow for a gi \Ten flood dis-

is greater than formerly.
development of flood-protection measures was of slow growth
and passed through four stages: (1) Removal of buildings to higher
ground, (2) impro\Tement of river-channel capacity with no clear idea
of the capacity required, (3) increase of channel capacity to equal the
measured peak flow of the most recent flood, and (4) construction
detention reservoirs to protect against the greatest
A brief discussion of these stages, in chronologie

charge
The

of storage or
probable floods
order, follows

first settlements were made along the streams, usually
the banks, and eneroaehed on the channels of seemingly
harmless streams. The outst,anding example of such encroachment
was in Denver. There Cherry Creek appeared to be such an inoffen-
si ve stream that some of the buildings, by a sort of gentlemen's
agreement among. property owners as to a reasonable width of channel,
encroached so far on the natural channel that they were supported
by stilts in the channel itself. The flood of 1864, which was the
first after settlement took place, showed the folly of such procedure'
and destroyed many of the encroaching buildings. 'Yhen rebuilt
they were placed outside the natural channel. Beyond that, little
was done to improve the channels. In Pueblo, similarly, buildings
erected as late as the eighties intruded in the na,tural ri vel' channel.
Fort Lyon was originally located on low ground close to the Arkansas
River, but after the flood of :May 1867 it was relocated 17 miles
westward on higher ground.
(2) As the various settlements grew, andlatcr floods caused over-
flow and resulting damage 'to them, steps were taken to increase
the channel capacity, chiefly by raising the bridges or reducing the
obstruction of the channel cross section caused by the bridges. But
this remedy was haphazard, as the volume of the floods was neither
measured nor ('stimated, and witholit such information it was not
possible'to know how much the channels should be widened or deep-
ciu\d in order to carry the probable volume without overflow.
(3) The second stage in channel improvement, and the third in
flood protection; consisted in measurement of the peak flow of each
outstanding flood as it occurred aIid the subsequent enlm'gement of
the <<hannel, if necessary, on the basis of this measured peak, to carry
future floods of as large volume.' In Pueblo, where the peak flow of
the Hood in ' 1894 was 39,000 second-feet, the chftl1ud th~oligh the

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The history of floods in Colorado spans the years from the early
days of the ninete('nth century, when the only whites in the region
were transient traders, trappers, and, travelers, through the middle
years of that century, when the early settlers arrived, to the present
day. Although severe floods occurred prior to settlement, the
evidence indicates that changes in the channels of at least some of
the streams subject to Doods have occurred since s(>ttlement b('gan.
The gradual de\Telopm(>nt of city flood-protection measures, as
experiences \\-ith floods have multiplied and mekorologic and hydro.
logic data have accumulated, can be easily traced.
Streams subject to severe floods are chiefly those on the ('astern slope
of the Rocky )'fountains, those in the plains area, cast of the moun-
tains, and those in the Rio Grande and San Juan Basins. 'When
settlement began, grent stretches of the plains area rast of the moun-
tains were covered with a natural growth of grasses indigenous to
the region, and, although varying with the sf'asons, it is believed
to ha \'0 bew fairly luxuriant. Tho stock-raising industry developed
so rapidly in the region and spread over it so widely as to cause too
dose grazing of the grass cover in many places. Thus the protection
the grass had afforded the soil from the viblent rains and resulting
floods was reduced materially. TlJ,e intensive grazing may have
tended also to pack, the soil, and by thus flugmcnting the' run-off,
increased the flood discharge in some parts of the area.
In some places streams seem to be widening as, the erosion of their
banks progresses. "Vhen Trinidad was first settled, about 1860,
the Purgatoire River was spanned by a footbridge about 30 feet long.
Floods since that time have enlarged the channel until it is now about
250 feet wide. The width of Fountain Creek at the town of Fountain
was about 40 feet in the nineties but has now increased to 200 feet.
Cherry Creek \vithin the present city limits of Den vel' has increased in
width from bout 14 to 100 feet sinoe the sixties. ' This process seems
to be continuing, and' some, floods so, erode the unprotected banks
of the streams that it is necessary to lengthen the bridges spalUling

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.. CHANNEL CAPACITY AND DEVELOPMENT
~ FLOOD-PROTECTION MEASURES

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them.
This tendency of streams to widen, at least locally, suggests erosion
accelerated by' soine pr~s incident to settlement' of the region bv
white people; but':the emnt to which it may beevidenee of greater
flood flows has not bc-en demonstrated. "
In some places, particUl.arlY in the South Platte and ArkansaS
Ri\Tei' Basins, the redudtionofchalUlel capacity has been' incident
to the development of irrigation. 'The manylli versions for irrigation
have reducedthe.r~ver flow to a mel'etrickle except during the period
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