Fire and Unglaciated Unconfined Channels
(Information compiled by G. David)
Poudre - Unglaciated
Unconfined Reach
(Photo by E. Wohl)
The expected response of unglaciated unconfined channels to fire depends largely on vegetation zones, 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 (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 dry ravel, 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 unconfined channels have larger floodplains where more sediment may be stored post-fire. The sediment may be stored over a large number of years in valley-fill and small alluvial fan deposits and persist through multiple fire cycles (Moody and Martin, 2001). Over the years the sediment may slowly be moved through the basin as vegetative re-growth ensues.
Increases in the peak flow, post-fire, may cause an increase in bank erosion and channel incision, especially in these unglaciated unconfined reaches where sediment shed from hillslopes may not reach the channel.