Fire and Unglaciated Confined Channels
(Information compiled by G. David)
North Saint Vrain Creek -
Unglaciated Confined Reach
(Photo by E. Wohl)
The expected response of unglaciated confined channels to fire depends largely on fire severity and rainfall intensity. The unglaciated channels are found in the lower ecotone and lower montane portions of the Front Range. The lower ecotone is made up of ponderosa pine forests and shrubland and the lower montane consists of Douglas fir and ponderosa pine forests. Historically, the stands at these lower elevations were much more open woodlands. After a period of logging and burning in the 19th century and a wet period in the early 20th century, the stands are now denser and even-aged (Romme et al., 2003; Macdonald and Larsen, in press). Moody and Martin (2001) found that a basin is most at risk for post-fire erosion and flooding in the first two to three years after burning. Summer convective storms in the unglaciated segments of the Front Range may cause the greatest increases in runoff and erosion post-fire.
Historically, the lower ecotone had low to moderate severity fires. Higher severity fires are expected after years of fire suppression have led to denser stands of trees. Low to moderate severity fires do not significantly change the runoff pathways and erosional processes in a basin and therefore are not expected to cause significant changes in the channel processes (Macdonald and Stednick, 2003). The high severity fires increase runoff from changes in soil properties and reduction of vegetation. Also, high severity fires increase erosion off hillslopes from rilling and channel incision.
Moody and Martin (2001) showed more sediment being shed from north-facing versus south-facing burned slopes. Intense convective storms are expected in the lower elevations, therefore a high severity fire followed by a few severe storms would lead to increased erosion on these slopes, particularly on north-facing slopes. The large storms coupled with a high severity burn area cause a greater amount of water and sediment to move through the basin. The unglaciated confined channels are more closely coupled with hillslopes and therefore would initially have sediment infill the channel post-fire. Over a number of years the sediment may slowly be moved through the basin as the channel incises once erosion off the hillslopes is reduced by vegetative re-growth.
Increases in the peak flow, post-fire, may cause an increase in bank erosion and channel incision. The effect of an increase in peak flow may be dampened by the larger volumes of sediment eroded into the channel.