WILDFIRE AND ITS FLUVIAL GEOMORPHIC IMPACTS TO MUDDY HOLLOW WATERSHED, POINT REYES NATIONAL SEASHORE

The 1995 Vision Fire in the Point Reyes National Seashore burned nearly 100% of the 8.3 km² Muddy Hollow Watershed. Annual rainfall ranges from 1016 mm at its 328 m–high peaks to 609 mm/yr at its Drake’s Estero outlet. Temporal and spatial geomorphic response of Muddy Hollow Creek, and its amount and intensity of rainfall and runoff was monitored for two years. The amount of large woody debris and bank failures existing prior to fire were compared to post fire amounts. Sediment deposition at the base of the watershed was quantified by analyzing numerous soil pits on a developing alluvial fan. Drainage density change was determined for a 0.28-km² headwater drainage.

Fire-adapted vegetation recovery was fairly rapid. The upper and lower watershed extremes had the earliest and most rapid geomorphic response to wildfire. During the first post fire winter, excessive runoff from granitic soils beneath the bishop pine-dominated headwaters was caused by development of a soil surface crust and hydrophobicity. Many 1st-order channels eroded headward, incising their small previously unchannelized fans that had formerly disconnected sediment and runoff from the main channel. Drainage density increased by 200% (not included were the temporary rill networks that developed across the uplands). Increased discharge initiated incision in the upland main channel, while high sediment supply and efficient sediment transport occurred through middle reaches with continuous riparian corridor. Channel filling, alluvial fan building, and subsequent braiding occurred on alder/ willow dominated lowlands. During the second winter, response in the uplands diminished as hydrophobicity disappeared and vegetation recovered, while the middle reaches significantly changed from bank erosion as abundant large woody debris from dying alders fell into the channel. Wood spacing changed from 17 m to 8 m, and eventually to 3 m. A minimum estimate of post fire sediment yield from the fan averaged 704-tonnes/km²/yr. Sediment supply was 2.7 times higher the second year than the first.

Post fire channel response did not follow the typical fire/flood scenario that characterizes coastal Southern California. Differences in response are likely due to differences in added apparent cohesion from dense root networks of California’s north coast vegetation.