1. Watershed Response to Fire Christine May Earth & Planetary Science

2. Severe fires can result in accelerated erosion by: 1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall 2.) heating and combusting soil organic matter 3.) burning logs that trap soil on steep slopes or store sediment in stream channels 4.) reducing the root strength of the soil

3. Rain Splash Fine sediment or ash dislodged by rain splash can clog soil pores causing surface sealing.

4. Severe fires can result in accelerated erosion by: 1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall 2.) heating and combusting soil organic matter 3.) burning logs that trap soil on steep slopes or store sediment in stream channels 4.) reducing the root strength of the soil

6. Rainfall Infiltration Subsurface flow Overland flow

7. Infiltration Rates Infiltration = the movement of water across the soil surface Influenced by Soil texture Ambient moisture

8. Time Infiltration Rate (mm/min) clay

9. Time Infiltration Rate (mm/min) clay sand

10. Overland Flow Surface erosion requires overland flow, which occurs when 1.) the rainfall rate exceeds the infiltration rate of the soil surface, or 2.) the soil is saturated

13. >4015-40<15 Runoff power index, slope (m/m) x relief: POSTFIRE REVEGETATION and RILL EROSION IN DEBRIS-FLOW BASINS

14. Hydrophobic Soils

15. A water-repellent layer of soil that prevents infiltration below that layer. Derived from plant material burned during a hot fire. The hydrophobic compounds (hydrocarbons) penetrate the soil surface as a gas and solidify after cooling, forming a waxy coating. Sandy soils with large pore spaces and areas with thick litter accumulations that experience very hot fires are especially susceptible.

16. Hydrophobic Soils (con’t) The thickness and continuity of hydrophobic layers varies, as does their persistence. Recovery: plant roots, soil microorganisms, and soil fauna help break up the hydrophobic layer. Negative feedback: reduced infiltration will decrease the amount of water available for plant growth and biological activity in the soil.

17. Field Tests Sprinkler experiments Infiltrometer Analysis: compare infiltration rates with rainfall rates from local raingages

18. Rehabilitation Measures On-site: revegetation –grass seeding – pitfalls? –straw mulch Off-site: sediment retention devices –straw bale check dams –directional log felling –sediment retention ponds

20. Photos by F.J. Swanson

22. [debris flow video]

25. Debris Flows Two initiation mechanisms: 1.) runoff-initiated - driven by low soil infiltration rates and the bulking of sediment detached by overland flow and surface erosion 2.) landslide-initiated - driven by soil saturation (requires high infiltration rates and low rooting strength)

26. Rill and channel erosion (1988 fire, 1989 storm, Yellowstone) Loss of root strength, saturation-failure of colluvium (1989 fire, 1997 storm, Idaho) Initiation of events through runoff and sediment bulking, early post-fire

27. Copyright © Tom Black 2002

28. 1989 debris flow-dominated event, NE Yellowstone

29. Initiation of events through loss of root strength, saturation and failure of colluvium (1989 fire, 1997 storm, central Idaho)

33. Years after forest removal / fire Relative root reinforcement From Ziemer 1981 Decay of dead roots

34. Years after forest removal / fire Relative root reinforcement From Ziemer 1981 Decay of dead roots Live roots

35. Years after forest removal / fire Relative root reinforcement From Ziemer 1981 Decay of dead roots Live roots 5 – 15 yrs

36. Probability of Occurrence Depends upon post-fire storm characteristics and the spatial pattern of high severity fire patches.

39. Temporal Patterns

40. Time since fire Likelihood of large-scale erosion Runoff-Dominated Surface Erosion Two-Phase Erosional Response

41. Time since fire Likelihood of large-scale erosion Runoff-Dominated Surface Erosion Saturation-Induced Slope Failures Two-Phase Erosional Response

42. Time since fire Likelihood of large-scale erosion Runoff-Dominated Surface Erosion Saturation-Induced Slope Failures Two-Phase Erosional Response 0 – 5 yrs 5 – 15 yrs

43. Post-Fire Rehabilitation Efforts for Debris Flows Suggestions from the class…

46. 0 to 30 yrs30 to 60 yrs 60 to 90 yrs > 90 yrs Time Since the Previous Debris Flow

48. Salvage Logging Dead wood can be an important element in sediment storage on steep hillslopes and in stream channels. Soil disturbance by logging operations and road construction can further accelerate erosion.

49. Short-term Patterns

50. Long-term Patterns

51. Event Reconstructions Studies that attempt to decipher long-term correlations among climate, fire, and erosion and their effects on landscape evolution using a variety of dating methods and evidence for past erosional events.

52. Copyright © Ron Dorn 2002

53. 1989 debris flow older fan sediments 1988 charred litter layer (burned soil surface) ‘Fire-related debris flows’

54. Yellowstone lodgepole: large, severe stand- replacing fires, RI 200-400 yr

55. Onset of the Little Ice Age (1200 AD) Meyer et al. 1992

56. Warmer millennial-scale periods

57. Cooler millennial-scale periods (terraces?)

58. Idaho batholith estimated mean sediment yields over different timescales (log scale)

60. Suspended sediment and bedload transport Watershed 3, HJA 0 4000 8000 12000 1956196219681974198019861992 t/km 2 /yr Suspended sediment Bedload Roads Patch cut Floods and debris flows

61. Questions?

62. Questions for the class: Is there evidence that fires preferentially travel through or burn hotter in steep, narrow valleys compared to planar hillslopes?

63. Questions for the class: How can information about erosion-prone areas be incorporated into: 1.) planning fuels treatment projects? 2.) wildfire management?

66. Central Idaho ponderosa: presettlement regime of light surface fires, RI 5-30 yr

67. Northern Hemisphere tree-ring temperature reconstruction (from Esper et al., 2002) Mann-Bradley-Hughes (1999) Esper et al. (2002) multiproxy tree-rings “Medieval Warm Period”“Little Ice Age” Severe fire, large debris flows both areas Frequent light fires Idaho; few fires Yellowstone Year AD

72. Conclusions Herbaceous revegetation reduces probability of post-fire runoff-generated events (maximum probability of occurrence 1-3+ yr after fire?) Later postfire saturation-induced failures –root strength control (maximum probability 4-10 yr after fire?) Geomorphic response to fire is transient but produces transient to persistent stream habitat alteration Climate is a strong control on fire regimes and associated geomorphic response, both in space and time, therefore… Fire-induced sedimentation is strongly episodic and variable at ~1000 yr timescales