Fire and Glaciated Unconfined Channels
(Information compiled by G. David)
Big Thompson River -
Glaciated Unconfined Reach
(Photo by E. Wohl)
The expected response of glaciated unconfined channels to fire depends largely on fire severity and rainfall intensity. The glaciated reaches are in the upper montane and subalpine vegetation zones where less frequent high severity fires are the norm. Moody and Martin (2001) found that most of the sediment after a fire came from the headwaters of a drainage basin and was subsequently stored in the higher order parts of the basin in flood plain deposits, alluvial fans and channel fill. The channel may eventually incise into these fill deposits, particularly if the flow in the channel is increased by a decrease in vegetative cover. Further downstream and in larger channels the effect of the fire may be reduced. The glaciated channels are at an elevation that is dominated by snowmelt rather than high intensity summer convective storms. The expected influence of fire on glaciated unconfined channels is separated by vegetation zones:
Upper Montane – The upper montane zone is characterized by a mixture of ponderosa pine and lodgepole pine forests. Both types of forest have dense stands in the Front Range. Fires in these forests are of mixed severity with patches of high severity. Studies have shown that high intensity summer convective storms are needed for significant increases in runoff and erosion in a basin post-fire (Macdonald and Stednick, 2003; Benavides-Solorio and Macdonald, 2005). Therefore, it is predicted that fires do not have as large an effect on glaciated stream channels. Most likely there is some increase in sediment and streamflow, but it may not be large enough to cause any drastic changes in the channel. Also, the glaciated unconfined channels are not closely linked to the hillslopes, therefore the effect of the fire may be even further reduced.
Subalpine – This zone is characterized by less frequent high severity fires. The return interval for fires in this zone can be greater than 100 years. Again, this area is dominated by snow rather than high intensity summer convective storms. Therefore, it is not predicted that these high intensity fires have a large influence on the water and sediment in these basins. Also, the glaciated unconfined channels are not closely linked to the hillslopes, therefore the effect of the fire may be even further reduced.