A shovel rests against a massive deposit of coarse sediment on the delta to Conconully Reservoir in Okanogan County, Washington. The sediment was deposited following the 2021 Muckamuck fire and a debris flow in July 2022. USGS scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire.
Post-wildfire sedimentation and release of metals to Conconully Reservoir, Washington Active
By Washington Water Science Center
August 22, 2023
The Issue:
Wildfires can negatively impact reservoirs and water supply. On August 4th, 2021, a lightning strike started the Muckamuck fire in Okanagan County, Washington. The fire burned approximately 13,297 acres (~21 square miles) of forest and shrubland before it was contained. The burn engulfed a large portion of the area which drains into the Conconully Reservoir, a popular location for fishing and recreation near the town of Conconully. The Reservoir also functions for flood control and irrigation water control. Wildfires and a history of mining in the area pose a risk to water quality and reservoir sedimentation.
How USGS will help:
The USGS will assess sediment and water quality in Conconully Reservoir and surrounding streams over the years following the 2021 fire. The USGS will quantify sedimentation rates and measure metal concentrations in sediment. This data will allow scientists to assess wildfire impacts on reservoir storage, water quality, and ecosystem health.
Problem
Increased sedimentation following wildfires can impact water quality, reservoir storage capacity, and aquatic habitat. Sedimentation concerns communities that rely on watersheds impacted by wildfire for irrigation, drinking water, and fishing. After wildfires, streamflow responds more rapidly to rainfall due to decreased vegetation cover and altered soil properties. Flash floods and erosion of sediment and ash increase the amount of material delivered to streams and reservoirs.
Sources/Usage: Public Domain. View Media Details
More water flowing to streams means that more sediment is carried into rivers and reservoirs. Post-fire landslides and debris flows may introduce even more sediment to rivers. Sediment that is deposited in rivers alters aquatic habitats, and sediment deposited in reservoirs reduces the water storage capacity in addition to degrading water quality.
Sediments generated by wildfire may contain high abundances of metals and toxic substances. Severe fires burn at high temperatures capable of releasing and redepositing metals and toxic polycyclic aromatic hydrocarbons (PAHs) from soils and local geology into surface water and sediment. Wildfires can also expose legacy mine waste that can be transported into stream, lakes, and reservoirs.
In 2021, a large wildfire burned about one-third of the North Fork Salmon Creek watershed that drains into the Conconully Reservoir. The West Fork Salmon Creek watershed also drains into the reservoir but only 6% of that area burned in the 2021 fire. Five wildfires had previously burned smaller portions of the watersheds. Additionally, the area has a history of mining for silver, gold, copper, zinc, and bismuth. The combination of previous wildfire history and mining within the watershed creates the potential for higher metal concentrations in reservoir sediment.
Objectives
The two primary objectives of this investigation are:
- Quantify reservoir sedimentation rates and potential increases in wildfire-induced reservoir sedimentation rates.
- Measure the concentration of metals in sediments and other parameters in Conconully Reservoir.
This project is one component of USGS work to assess post-fire risk in California and Washington.
Relevance and Benefits
Current USGS research is aimed at better tracking and prediction of water supply and quality responses to wildfires in watersheds throughout the western U.S. and improved understanding of the interactions of wildfire responses with other factors such as land and water use changes, climate change and drought extent and intensity. This study specifically assesses the impact of wildfires on sedimentation and water quality with implications for reservoir storage capacity and ecosystem health.
This study will provide a comparison of background and fire-induced sedimentation rates from past wildfires in the basin and the potential duration of recovery trajectories for increased sedimentation following wildfire. The results can be used to inform the management of watersheds in preparation and response to wildfires regarding the potential duration of stream sedimentation as well as the management of Conconully Reservoirs. Furthermore, this work will provide an understanding of the potential of wildfire-induced release of toxic metals from regional geologic materials that can contaminate reservoirs and river corridors with water and sediment toxic to humans, fish, and other wildlife.
Additionally, the examination of potential downstream impacts of post-fire sedimentation complements ongoing regional efforts to characterize and rehabilitate fish habitat in the Okanagan River and greater Columbia River basins. Sedimentation relates to impacts on fish habitat and health. This research can be used to infer potential sedimentation resulting from other wildfires in the region, which will inform future preparedness, study design, and response to wildfires.
Approach
Sediment cores extracted from Conconully Reservoir will be used to characterize background sedimentation rates and quantify fire-induced sedimentation. Extraction of sediment cores in two inlets to the reservoir and within the deepest area of the reservoir will allow for the comparison of sedimentation from historical wildfires that have burned different portions of the North Fork and West Fork Salmon Creek basins using 210Pb dating. Counts of charcoal fragments in sediment cores will be completed to identify periods of post-fire sedimentation that can be compared with documented fire history in the basin.
Sources/Usage: Public Domain. View Media Details
A bathymetric survey will be compared to prior bathymetry to characterize reservoir sedimentation and examine the potential impact of a recent debris flow initiated in the Muckamuck Fire burn area on reservoir sedimentation.
The investigation will quantify wildfire impacts on stream water and sediment quality examining the spatial extent and concentration of metals and potentially toxic trace elements. Additionally, it will determine the bioavailability of these elements in stream flow, reservoir porewaters, reservoir sediment, and soil from burned and unburned areas.
Sediment deposited in delta of Conconully Reservoir, Washington
A shovel rests against a massive deposit of coarse sediment on the delta to Conconully Reservoir in Okanogan County, Washington. The sediment was deposited following the 2021 Muckamuck fire and a debris flow in July 2022. USGS scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire.
Bank sediment exposure near the Conconully Reservoir, Washington
A shovel leans against the bank of the Conconully Reservoir in Okanogan County, Washington. Layers of legacy sediment are visible in the bank exposure.
A shovel leans against the bank of the Conconully Reservoir in Okanogan County, Washington. Layers of legacy sediment are visible in the bank exposure.
Collecting sediment samples after wildfires in Washington State
A sediment sample collection bag and jar sit on soils burned in the 2021 Muckamuck wildfire. Scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire. The high temperatures of severe wildfires are capable of releasing and redepositing metals on the soil surface.
A sediment sample collection bag and jar sit on soils burned in the 2021 Muckamuck wildfire. Scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire. The high temperatures of severe wildfires are capable of releasing and redepositing metals on the soil surface.
USGS scientist measures bathymetry with a remotely controlled Q-boat on Conconully Reservoir, Washington
linkUSGS Hydrologic Technician Chaska Boucher operates a remotely controlled Q-boat on Conconully Reservoir in Okanogan County, Washington. A portion of the watershed burned in the 2021 Muckamuck wildfire and a debris flow occurred in 2022.
USGS scientist measures bathymetry with a remotely controlled Q-boat on Conconully Reservoir, Washington
linkUSGS Hydrologic Technician Chaska Boucher operates a remotely controlled Q-boat on Conconully Reservoir in Okanogan County, Washington. A portion of the watershed burned in the 2021 Muckamuck wildfire and a debris flow occurred in 2022.
The Issue:
Wildfires can negatively impact reservoirs and water supply. On August 4th, 2021, a lightning strike started the Muckamuck fire in Okanagan County, Washington. The fire burned approximately 13,297 acres (~21 square miles) of forest and shrubland before it was contained. The burn engulfed a large portion of the area which drains into the Conconully Reservoir, a popular location for fishing and recreation near the town of Conconully. The Reservoir also functions for flood control and irrigation water control. Wildfires and a history of mining in the area pose a risk to water quality and reservoir sedimentation.
How USGS will help:
The USGS will assess sediment and water quality in Conconully Reservoir and surrounding streams over the years following the 2021 fire. The USGS will quantify sedimentation rates and measure metal concentrations in sediment. This data will allow scientists to assess wildfire impacts on reservoir storage, water quality, and ecosystem health.
Problem
Increased sedimentation following wildfires can impact water quality, reservoir storage capacity, and aquatic habitat. Sedimentation concerns communities that rely on watersheds impacted by wildfire for irrigation, drinking water, and fishing. After wildfires, streamflow responds more rapidly to rainfall due to decreased vegetation cover and altered soil properties. Flash floods and erosion of sediment and ash increase the amount of material delivered to streams and reservoirs.
Sources/Usage: Public Domain. View Media Details
More water flowing to streams means that more sediment is carried into rivers and reservoirs. Post-fire landslides and debris flows may introduce even more sediment to rivers. Sediment that is deposited in rivers alters aquatic habitats, and sediment deposited in reservoirs reduces the water storage capacity in addition to degrading water quality.
Sediments generated by wildfire may contain high abundances of metals and toxic substances. Severe fires burn at high temperatures capable of releasing and redepositing metals and toxic polycyclic aromatic hydrocarbons (PAHs) from soils and local geology into surface water and sediment. Wildfires can also expose legacy mine waste that can be transported into stream, lakes, and reservoirs.
In 2021, a large wildfire burned about one-third of the North Fork Salmon Creek watershed that drains into the Conconully Reservoir. The West Fork Salmon Creek watershed also drains into the reservoir but only 6% of that area burned in the 2021 fire. Five wildfires had previously burned smaller portions of the watersheds. Additionally, the area has a history of mining for silver, gold, copper, zinc, and bismuth. The combination of previous wildfire history and mining within the watershed creates the potential for higher metal concentrations in reservoir sediment.
Objectives
The two primary objectives of this investigation are:
- Quantify reservoir sedimentation rates and potential increases in wildfire-induced reservoir sedimentation rates.
- Measure the concentration of metals in sediments and other parameters in Conconully Reservoir.
This project is one component of USGS work to assess post-fire risk in California and Washington.
Relevance and Benefits
Current USGS research is aimed at better tracking and prediction of water supply and quality responses to wildfires in watersheds throughout the western U.S. and improved understanding of the interactions of wildfire responses with other factors such as land and water use changes, climate change and drought extent and intensity. This study specifically assesses the impact of wildfires on sedimentation and water quality with implications for reservoir storage capacity and ecosystem health.
This study will provide a comparison of background and fire-induced sedimentation rates from past wildfires in the basin and the potential duration of recovery trajectories for increased sedimentation following wildfire. The results can be used to inform the management of watersheds in preparation and response to wildfires regarding the potential duration of stream sedimentation as well as the management of Conconully Reservoirs. Furthermore, this work will provide an understanding of the potential of wildfire-induced release of toxic metals from regional geologic materials that can contaminate reservoirs and river corridors with water and sediment toxic to humans, fish, and other wildlife.
Additionally, the examination of potential downstream impacts of post-fire sedimentation complements ongoing regional efforts to characterize and rehabilitate fish habitat in the Okanagan River and greater Columbia River basins. Sedimentation relates to impacts on fish habitat and health. This research can be used to infer potential sedimentation resulting from other wildfires in the region, which will inform future preparedness, study design, and response to wildfires.
Approach
Sediment cores extracted from Conconully Reservoir will be used to characterize background sedimentation rates and quantify fire-induced sedimentation. Extraction of sediment cores in two inlets to the reservoir and within the deepest area of the reservoir will allow for the comparison of sedimentation from historical wildfires that have burned different portions of the North Fork and West Fork Salmon Creek basins using 210Pb dating. Counts of charcoal fragments in sediment cores will be completed to identify periods of post-fire sedimentation that can be compared with documented fire history in the basin.
Sources/Usage: Public Domain. View Media Details
A bathymetric survey will be compared to prior bathymetry to characterize reservoir sedimentation and examine the potential impact of a recent debris flow initiated in the Muckamuck Fire burn area on reservoir sedimentation.
The investigation will quantify wildfire impacts on stream water and sediment quality examining the spatial extent and concentration of metals and potentially toxic trace elements. Additionally, it will determine the bioavailability of these elements in stream flow, reservoir porewaters, reservoir sediment, and soil from burned and unburned areas.
Sediment deposited in delta of Conconully Reservoir, Washington
A shovel rests against a massive deposit of coarse sediment on the delta to Conconully Reservoir in Okanogan County, Washington. The sediment was deposited following the 2021 Muckamuck fire and a debris flow in July 2022. USGS scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire.
A shovel rests against a massive deposit of coarse sediment on the delta to Conconully Reservoir in Okanogan County, Washington. The sediment was deposited following the 2021 Muckamuck fire and a debris flow in July 2022. USGS scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire.
Bank sediment exposure near the Conconully Reservoir, Washington
A shovel leans against the bank of the Conconully Reservoir in Okanogan County, Washington. Layers of legacy sediment are visible in the bank exposure.
A shovel leans against the bank of the Conconully Reservoir in Okanogan County, Washington. Layers of legacy sediment are visible in the bank exposure.
Collecting sediment samples after wildfires in Washington State
A sediment sample collection bag and jar sit on soils burned in the 2021 Muckamuck wildfire. Scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire. The high temperatures of severe wildfires are capable of releasing and redepositing metals on the soil surface.
A sediment sample collection bag and jar sit on soils burned in the 2021 Muckamuck wildfire. Scientists are studying the sediment and water quality in rivers and reservoirs impacted by the large fire. The high temperatures of severe wildfires are capable of releasing and redepositing metals on the soil surface.
USGS scientist measures bathymetry with a remotely controlled Q-boat on Conconully Reservoir, Washington
linkUSGS Hydrologic Technician Chaska Boucher operates a remotely controlled Q-boat on Conconully Reservoir in Okanogan County, Washington. A portion of the watershed burned in the 2021 Muckamuck wildfire and a debris flow occurred in 2022.
USGS scientist measures bathymetry with a remotely controlled Q-boat on Conconully Reservoir, Washington
linkUSGS Hydrologic Technician Chaska Boucher operates a remotely controlled Q-boat on Conconully Reservoir in Okanogan County, Washington. A portion of the watershed burned in the 2021 Muckamuck wildfire and a debris flow occurred in 2022.