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Research Feature

Basin morphology and fluvial landforms alter the downstream transport and fate of post fire sediment associated phosphorus potentially reducing risk to water supply

Key messages

• Natural and anthropogenic disturbance in forested Canadian landscapes underlain by deposits of potentially erodible fine sediment pose a significant risk for increased vulnerability to water treatment due to the mobilization and transfer of fine sediment and associated phosphorus as well as dissolved organic carbon (DOC) to receiving waters.
  
  
• Deposition of particulate and dissolved fractions of pyrogenic materials in reservoirs can act as an internal source of phosphorus (P) when released to the water column that can lead to the proliferation of algal productivity. 
  
  
• The extent and severity of impact on water quality from harvested and wildfire impacted landscapes will depend on several factors including vegetation type, hydroclimate, basin physiography and alluvial form, the type and severity of landscape disturbance as well as the availability of fine sediment sources.
  
  
• Changes in phosphorus form, mobility and transport are most elevated in burned, high elevation landscapes with high slopes such as the Montane Cordillera. However, in high-elevation landscapes with U-shaped valleys, lower hillslope-stream channel connectivity and natural sinks for sediment storage such as broad floodplains and alluvial fans attenuate the propagation of fine sediment and can significantly reduce the impacts of landscape change on water quality and downstream water treatability.
  
  
• Impacts of forested landscape disturbances on water treatability require the implementation of strategic water quality monitoring to advance assessment and prediction of wildfire impacts on water supplies. Strategically monitored water-quality data focused on parameters that are critical to human and (or) ecosystem health, relevant to water-treatment processes and drinking-water quality, and (or) inform the role of precipitation and discharge on flow paths and water quality are most useful. Forest harvesting companies have a role to play by improving the resilience of forested landscapes to climate change (drought, wildfire) on water treatability.
  
  

Water Treatment Costs Reduced by

per cent

Summary

Wildfire is amongst the most severe landscape disturbance types in forested source water regions. The post-fire delivery of sediment and associated phosphorus (P) to receiving waters is typically increased and its impacts on water quality, aquatic habitat, and water treatability are widely acknowledged. The severity and duration of these impacts are influenced by factors such as the severity and intensity of wildfire, vegetation type, hydroclimate, and geology. However, the degree of hillslope-stream connectivity along the river continuum which is a key driver of pyrogenic materials to receiving streams is also controlled by basin morphology and fluvial landforms in the stream corridor. In some watersheds these two landscape features can alter the storage and fate of pyrogenic materials thereby potentially reducing the downstream risk to drinking water reservoirs.

In high sloping forested regions such as the eastern slopes of the Rocky Mountains in Alberta, streams are highly connected to hillslopes and the post fire delivery of bioavailable particulate P to streams can persist for decades (Emelko et al., 2016) and these pyrogenic materials can be propagated long distances downstream (Stone et al., 2014; Stone et al., 2021).
 

Additionally, the ingress and subsequent storage of these materials in receiving gravel bed streams (Maltauro et al., 2024) can influence the soluble P concentration in the water column and bed sediments (Watt et al., 2021) and promote biostabilization of river bottom sediment that when eroded can lead to more variable water quality (Stone et al., 2011).

In contrast, forested regions with extensive flood plains such as those in Waterton National Park, Alberta are poorly connected to streams, the delivery of P to streams is reduced, and bioavailable particulate P content in these rivers decline within a few years after wildfire (Tullio, 2022). In these environments the storage of pyrogenic materials and associated P is strongly controlled by fluvial landforms that include floodplains, gravel bed rivers, and alluvial fans which have the potential to reduce the risk to downstream water quality by trapping and storing pyrogenic materials and associated P (Meyer and Wells, 1997; Pegler, 2022). However, there is uncertainty about the storage time and legacy of these deposits in the face of climate change and extreme weather where elevated rainfall totals and intensities may remobilize these materials and have potentially serious implications for water quality—thus creating longer term challenges to water supply.

Publications

Emelko, M.B., Stone, M., Silins, U., Allin, D., Collins, A.L., Williams, C.H., Martens, A.M. and Bladon, K.D., 2016. Sediment‐phosphorus dynamics can shift aquatic ecology and cause downstream legacy effects after wildfire in large river systems. Global Change Biology, 22(3):1168-1184.

Maltauro, R., Stone, M., Collins, A.L. and Krishnappan, B.G., 2024. Evaluating effective particle size distributions of cohesive sediment under varying shear stress and bed configurations in a rotating annular flume. Water, 16(4):546.
Meyer, G.A. and Wells, S.G., 1997. Fire-related sedimentation events on alluvial fans, Yellowstone National Park, USA. Journal of Sedimentary Research, 67(5):776-791.

Pegler, S., 2022. Can alluvial landforms attenuate post-wildfire lake sedimentation rates and external phosphorus inputs? (Master's thesis, University of Waterloo).
Stone, M., Emelko, M.B., Droppo, I.G. and Silins, U., 2011. Biostabilization and erodibility of cohesive sediment deposits in wildfire-affected streams. Water Research, 45(2):521-534.

Stone, M., Collins, A.L., Silins, U., Emelko, M.B. and Zhang, Y.S., 2014. The use of composite fingerprints to quantify sediment sources in a wildfire impacted landscape, Alberta, Canada. Science of the Total Environment, 473:642-650.

Stone, M., Krishnappan, B.G., Silins, U., Emelko, M.B., Williams, C.H., Collins, A.L. and Spencer, S.A., 2021. A new framework for modelling fine sediment transport in rivers includes flocculation to inform reservoir management in wildfire impacted watersheds. Water, 13(17):2319.

Tullio, M., 2022. Impact of the Kenow wildfire on the form and mobility of particulate phosphorus in gravel-bed rivers at large basin scales: Implications for downstream propagation (Master's thesis, University of Waterloo).

Photo: Working onsite with partner X.

Contributors

UWaterloo: Fariba Amiri, Monica B. Emelko, Ryan H.S. Hutchins, Steven Ngo

UofA: Alyssa K. Bourgeois, Kevin J. Devito, Erin Cherlet, Chad W. Cuss, David Olefeldt, Julia Orlova, Uldis Silins, Suzanne E. Tank, Lauren M. Thompson, Chris H.S. Williams

Trent U: Jim M. Buttle

VIU: William C. Floyd

DalhousieU: David E. Foster, Rob Jamieson

UBC: Mark S. Johnson, Hannah J. McSorley

NRCan: Kara L. Webster