FLOOD06408

• County: Weld
• Community: Windsor
• Stream Name: Big Thompson/South Platte/Cach la Poudre
• Basin: South Platte
• Title: Drainage Basins and Areas of Past Flooding in the Winsor Area, Colorado
• Date: 1/1/1971
• Prepared For: University of Colorado
• Prepared By: James Paul Walsh, Jr.
• Floodplain - Doc Type: Flood Documentation Report

9

Techniques for Mapping Past Floods

The maps of past flooding are the product of several different

sources of information (Table 2). The primary source was stereo-air

photography where, topography, sediments and'vegetation (Fig. 1) con'-

stituted the criteria used for defining flood, bo.pndaries. Supplemen-

tary information came from field checks, eye witness accounts,and

previous work on quaternary stratigraphy' and soils, and early

topographic map,s.
Topography was the most frequently use4 criterion for mapping,

Flat areas along channels ",<,re mapped as areas of past flooding

whose outer limits occurred at the first topographic break (Fig. lA).

Along channels with terraces the upper limit of the lowest terrace

was mapped as the flood boundary (F.ig, IE). \mere streams flow' in

arroyos rather than on flood plains, the edges of the an,oyos were

taken as the limits to flooding (Fig, lC).

Unvegetated sedimen,t on valley bottoms was tak.en as evidence

of recent flooding .(Fig. ID). Flood boundaries were drawn at the

outer limit of unvegetated sediment.

Vegetation was occasionally useful as a criterion for mapping

past floods (Fig. lE). In areas ,of high watEr tab~e, undisturbed

vegetation was found to be a thick growth of sedges, grasses and

willows, which a'ppears darker in air photos than the surrounding

sparse vegetation.

w~ere flood criteria were vague, boundaries of past floods

were drawn to confonn to the observations of local residents. The