Mike Peron/ Tahoe Daily Tribune
SOUTH LAKE TAHOE — With the Caldor Fire having burned over 217,000 acres in and around the Tahoe basin, there is concern over the ongoing impact of the Caldor Fire on the clear blue water of Lake Tahoe.
Funding coordinated by the Tahoe Science Advisory Council and sponsored by several non-profits and funding agencies, has allowed a team of researchers to closely monitor the immediate impact of ash and smoke on the highly prized clarity of Lake Tahoe. The increased frequency of water sample testing and depth visibility measurements will not only help understand the current impact on Lake Tahoe, but also how future fires may affect other lakes and reservoirs in Sierra Nevada.
The clarity of the lake is sensitive to the input of fine sediment as well as algae growth stimulated by nitrogen and phosphorus. In recent measurements, clarity has been reduced by over 10 feet, with contributing factors including ash deposits and reduced sunlight caused by smoke.
“Some days clarity will be reduced ten feet then a week later it is back to what I would call ‘normal’ even though normal is highly variable” said Dr Geoffrey Schladow, Director of the UC Davis Tahoe Environmental Research Center (TERC), indicating that we will have to wait to see more data to better understand the relative contribution of the various factors leading to reduced clarity and how sustained they will be. “Maybe in a couple of months we will be able to see what the short term effect is and it may take us until this time next year to know the long term effects.”
With the data not fully analyzed, the scientists have already seen anecdotal evidence of large algae growth which is stimulated by nutrients (nitrogen and phosphorus) provided by the ash. Meanwhile, the reduced sunlight allows UV-avoiding plankton to move closer to the surface and feed off the algae supply. A similar phenomenon was observed after the Angora fire, however due to temperate weather in the months following the fire, the lake was able to settle and return to normal levels of primary productivity (algae growth) within a couple of months.
However, the Caldor Fire has already burned more than 70 times the area of the Angora Fire and the nearby Dixie and Tamarack Fires have also contributed smoke and ash in the region which may result in more significant and sustained impact on lake clarity and algal growth.
“We’re going to keep having fires” said Dr Schladow, underscoring the importance of continued research to understand the impact of fires on our water resources. With limited past examples, each time an event like the Caldor Fire occurs, research is an important step in continuing to learn and prepare for the inevitable next fire.
The Tahoe Science Advisory Council will be holding a meeting on September 16 to discuss further research funding priorities around the Caldor Fire beyond the current examination of the impact of smoke and ash on Lake Tahoe. They are looking to coordinate additional research projects related to the impact of the Caldor Fire and possibly around the efficacy of preventative measures such as prescribed burns and vegetation management.
As the ash settles and the smoke clears, there are additional questions around potential knock-on effects and further disruptions to the Lake’s clarity due to erosion in surrounding watersheds.
With the Caldor Fire having spread in to the Tahoe basin, there is also a big concern that erosion and runoff in the burned areas will introduce soil and ash particles to the Lake. The topography of the area burned is a key component in the role erosion will play in lake clarity. The Gondola Fire in 2002 included more steep hills close to the lake than the Angora Fire leading to a more extensive degradation of water clarity. The Caldor Fire burned steep hills, however they are further away from the lake so a direct comparison cannot be drawn in assessing the magnitude of risk.
Given the intensity of the Caldor Fire, there is also concern of a hydrophobic (water repelling) soil layer forming. A hydrophobic layer is formed by gasses released from burning conifer foliage, penetrating the soil and solidifying in to a waxy layer. This lessens the soils ability to absorb water reducing availability for plant growth and increasing risk of runoff of topsoil and ash in to streams. Depending on the thickness of the layer, water repellency can last more than 3 years.
Rivers and creeks which have been treated to reintroduce meanders and improve the connection between the flood plain and channel have a stronger ability to naturally filter out sediment and nutrients. Large areas of the watersheds of Trout Creek and Upper Truckee River burned in the Caldor Fire, but significant channelized reaches of both streams had been restored prior to the fire, and it is hoped this will reduce some of the impact on these streams. However, the creeks ability to filter out sediment will be limited should there be a significant input of material caused by extreme weather and subsequent erosion.
Mike Peron/ Tahoe Daily Tribune
“Each fire is unique and it depends not only on the specifics of the fire, but the weather patterns that follow it.” Dr Alan Hayvaert of the Desert Research Institute.
Should we see heavy rains in the Fall, there could be significant transport of sediment resulting in damage to structures downhill from burn areas in addition to a flow of material into the waterways and Lake Tahoe.
“Large debris flows following fires can be like liquid avalanches,” said Dr Hayvaert, indicating there is “concern for the landscape, particularly in the west where ground cover tends to be more sparse.”
The US Forest Service’s Burned Area Emergency Response (BAER) program takes steps to mitigate erosion and protect watersheds from further contamination. These steps can include installation of erosion control devices, planting for erosion control and application of debris stabilizers. Following Angora, the US Forest Service took preventative measures and there was limited precipitation in the Fall and Winter so there was not a significant amount of sediment in watersheds that could be conclusively linked to the Angora Fire.
“Early precipitation is key,” TERC Research Associate Robert Coats, “Early snow would protect from immediate erosion. But due to climate change we are moving towards rain in the Fall and early and rapid snowmelt.” The changing climate is cause for concern not only in the increased risk of fires due to periods of drought, but also for the extreme weather patterns that can be particularly destructive following a fire.
Significant erosion this fall and winter would lead to further short-term decline in Lake Tahoe’s clarity and provide additional nutrients over the long run for algae growth. We could also see a spike in invasive plants. Fortunately, researchers will continue to monitor the conditions and the ongoing effects seen in Lake Tahoe.
Alison Waldron/ Tahoe Daily Tribune
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