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<br />Inlerred from damage to vegetation). Bucket data and
<br />nearby systematic gaged data are used to correlate
<br />rainfall data with geomorphic evidence accounting lor
<br />variabllity such as soil type and cohesiveness,
<br />vegetation cover, and hUlslope gradient and length.
<br />Then, variations In the geomorphic evidence In the
<br />storm Blll8 are used to estimate a ralnlall amount where
<br />geomorphic data are available. Finally, an isohyetal
<br />map Is drawn considering available data. Sometimes,
<br />storm path can be Inlerred lor recent storms.
<br />In the second or hydrologic method, Indirect
<br />estimates 01 peak discharge can be obtained lor many
<br />srnaII basins in the area aIIected by the storm, The
<br />emphasis is obtaining peak-dlscharge data lor small
<br />basins (<1-3 km2) where spatial and temporal rainfall
<br />variabiflty Is assumed to be small, thus, providing more
<br />reliable paleohydrologic rainfall estimates. High-water
<br />mar1<s (HWMs) 01 recent noods or paJeostage indicators
<br />(essentially old HWMs) lor paleofloods, channel
<br />geometry, and hydraulic data for a stream are used to
<br />estimate peak discharge such as with the critlcal-depth
<br />method (Jarrett, 1991; Jarrett and Tomlinson, 20(0).
<br />RaInfall-runoff( (RF-RO) modeling with physical basin
<br />attributes also can be used to derlve Independent
<br />estimates of rainfall for each small basin. RF-RO
<br />modeling Is used to back-calculale rainfall Intensity and
<br />amounts from the peak dischaJge and a description of
<br />basins. Umlted space precludes Including RF-RO model
<br />results. Hydrologic rainfall estimates are used to: (1)
<br />help draw the IsohyetaI map with available geomorphic
<br />rainfall estimates; (2) develop isohyetal maps for
<br />historic and prehistoric rainstorms; (3) compare results
<br />with other independent sources 01 rainfall data or; (4)
<br />provide ralnlall estimates if no other source exists,
<br />
<br />4. RESULTS
<br />
<br />No systematic precipitallon or streamflow monitoring
<br />existed in the Bullalo Creek bumed area in 1996, On
<br />July 15-16, 1996 (before addltional rainstorms), data
<br />collection consisted 01 obtaining rainfall bucket data,
<br />peak now, and paleohydrologic data lor the area.
<br />Eleven ralntall bucket observations were available from
<br />residents who had various types of plastic rain gages,
<br />MaxImum ralnlall for the July 12, 1996 storm was about
<br />eo mm in the community of BuIIaIo Creek and
<br />headwaters of Spring Creek (fig. 1 a); residents stated
<br />most 01 the rain feU about 2000 to 2100 MDT,
<br />The amount and location of Ir8sh rill and gully erosion
<br />on hillslopes generally less than 5-10 m In length was
<br />compared to nearby rainfall amounts lor the July 12th
<br />storm. HilsIopes (burned or unbumed with sparse
<br />vegetation) with less than about 25 mm rain (bucket
<br />data) had some sedment movement and minimal rill
<br />development. HlIlsIopes thet received about 50 mm of
<br />rain typically had rills about 7S mm deep and 50 mm
<br />wide. Hillslopes that received about 7S mm of rain
<br />typically had extensive rilling and numerous gullies up to
<br />0.5 m deep and a meter wide. HllIslope erosion in areas
<br />
<br />without any bucket data then was used to estimate
<br />rainfall in areas lrom these general relations. Numerous
<br />gullies up to a meter deep and 3 m wide, very extenslve
<br />rilling, and large headcuts were documented In an area
<br />about 2,5 \un southeast of Bullalo Creek near the
<br />headwaters 01 Sand Draw. Spring Creek, Shlnglemlll
<br />Creek, and Spring Gulch. This area of maximum erosion
<br />was used 10 Infer the location and area of maximum
<br />storm rainfall amount 01 at least 110 mm (fig. 1 a).
<br />Large quantitles 01 sediments were mobilized on
<br />hUlsIopes and In channels in the burned area c1lring the
<br />July 12th storm. A dstInct black, bum boundary (line)
<br />on rocks defined pre-1Iood ground surfaces and was
<br />used as a reference to estimate the general surface
<br />erosion from sheetwash (overland flow), Care was
<br />taken to estimate general erosion rather than the local
<br />erosion around a rock, In addition, pillars of soil ware
<br />preserved under some surface rocks and metal objects
<br />on the burned areas, The area 01 maximum sheetwash
<br />also was limited to the headwalers of Shinglemil Creek,
<br />Spring Gulch, Sand Draw, and Spring Creek.
<br />Sedments moved on hillslopes ranged from silt to
<br />cobble-sized material, and 2,5-m diameter bouiders
<br />were transported In some channels. A large amount 01
<br />the flood-transported sediment was daposlted as
<br />aDuvlal fans. Many new fans had dimensions 01 about
<br />100 m x30 m x 1,5-2 m such as In Sand Draw, Spring
<br />Creek, ShInglemlll Creek, and Spring Gulch, Well
<br />preserved. fresh tributary fans on the Buffalo Creek
<br />floodplain were used to inler relative ftood timing, and
<br />that the storm moved easterly (downstream), which
<br />likely exacerbated ftooding.
<br />In unburned vegetatad areas, rainfall also was
<br />Inlerrad by the amount of dull that flcated and was
<br />repositioned by sheetwash, RaInfall less than about 50
<br />mm (depending on dull thickness and composition)
<br />partfally ftoaIed and reoriented needles, twigs, and other
<br />elongated debris perpendicular to the now directlon and
<br />spaced about 2 to 4 em distance apart. These micro-
<br />scale leatures appear 10 have functloned as small dams
<br />ponding rainlall and hindering raInfa/I runoff. RaInfall
<br />greater than about 50 to 75 mm produced a cascading
<br />laIlure 01 these small debris darns. Small channels were
<br />preserved within the dull (similar to channellnclslon),
<br />which enabled peak discharge estimation. These
<br />features are slmllar to log jams in rivers such as the
<br />1982 Lawn Lake dam failure in Rocky Mountain NatIonal
<br />Park, located about 100 km nol1hwest 01 Denver,
<br />Colorado (Jarrett and Costa, 1986).
<br />Peak-flow estimates for twenty streams, ranging In
<br />size from about 0.1 kJn2 to the total burned area of about
<br />50 kJn2, also were used to help Identify areas of
<br />maximum rainfall, These basins have varied
<br />characteristics such as vegetation cover, bum intensity
<br />(InclUdIng no bum). watershed aspect and slope, and
<br />sediment sizes, A number of severely-bumed small
<br />basins In areas 01 maxlmum rainfall had unit discharges
<br />(peak cIlscharge divided by drainage area) from about 45
<br />to 60 m3/slkm2. [For comparison. the maximum unit
<br />
<br />JOllfT SESSIONS
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