FLOOD09816

• County: Statewide
• Basin: Statewide
• Title: Forest- Fire Devegetation and Drainage Basin Adjustments in Mountenous Terrain
• Date: 1/1/1977
• Prepared By: Department of Geology University of New Mexico
• Floodplain - Doc Type: Educational/Technical/Reference Information

Forest-Fire Devegetation and Basin Adjustments

hydrophobic layer on the gentler slopes of Burnt Mesa study area. and more
extensive remnants of the layer are still preserved on the hillslopes there
(fig. 5).

In the late fall of 1977. another change in ground conditions and weather-
ing processes resulted in an increase in sediment availability and erosion.
Freeze-thaw cycles began in November 1977 as available soil moisture was sub-
jected to diurnal fluctuations in surface temperature. Freezing and thawing
disrupted the previous ground-surface texture (figs. 7A and 7B). Frost
heaving resulted in the upward movement of fine particles relative to coarser
ones. Loose. fine-grained sediment was concentrated at the ground surface
where minor runoff events could easily remove it. A three- to eight-fold
increase in erosion occurred between October and December. 1977. in response
to this surface alteration (fig. 8).

Differential rates and amounts of erosion over the watershed are most
likely related to differences in soil moisture and consequently, frost-heaving
intensity. Variations in erosion on the hillslopes are common along erosion-
pin transects; however. all transects showed a dramatic increase in erosion
during the freeze-thaw cycles (fig. 8). The intense-burn watershed of
Burnt Mesa experienced the greatest amount of erosional response to frost
heaving (fig, 8). This is attributed to (1) hillslope aspect (orientation) and
moisture regimes. (2) relative proportion of fine sediment in the colluvial
cover. and (3) lack of ,extensive vegetational cover.

Seasonal variations in the erosion of the Burnt Mesa watersheds are given
in table 4 and figure 6. Erosion rates increased significantly during late fall
and spring. These seas~ns coincide with periods of accelerated mechanical
weathe!ing (frost heaving:). Erosional stability and periods of low sediment
production occur when the watersheds are blanketed with snow (fig. 6).

Other Factors Controlling Watershed Erosion

Erosion measurements given in table 4 suggest that factors other than
mechanical weathering inf}uence sediment supply. For example. erosion rates
are greater and more variable in the Burnt Mesa area than in the Apache
Springs area (table 4). Surface denudation in the Apache Springs area is
distributed more uniformly through time than in Burnt Mesa (fig. 6). Rates
of revegetation of the watersheds are partially responsible for these varia-
tions in erosion. Grass quickly re-established cover over a greater part of
the surface in the Apache Springs area than in the Burnt Mesa area. Re-
vegetation reduced the amount and variation of surface erosion over time be-
cause m~chanical weathering was less ~ffective in the vegetated zon\~s.

Anotl:er vegetational factor influencing the variability of erosion through
time is the post-fire needle cast . A needlecast is a thin blanket of pine
needles covering the grDund surface which forms when partially-burned trees
drop dead needles to the ground (fig. ZB). Needlecasts are common in
moderate and light burns. Their occurrence depends on the density of
standing conifers. arid of dead. unburned needles. This type of forest
litter protects the recently devegetated colluvial cover and reduces erosion
(Connaughton, 1935; Megahan and Molitor. 1975). Needlecasts break the im-
pact of raindrops and distribute rainfall over the surface. allowing slow in-

211