The Mokelumne River has been running brown with sediment after each storm Calaveras and Amador Counties have seen this winter. For those who drive between Mokelumne Hill and Jackson, the brief glimpse of the river from the Highway 49 bridge provides a hint of what is trickling down from tributary streams and off the burned banks upstream.
Since the Butte Fire was contained October 1, optimistic weather reports began spreading news of a huge El Nino event for the western United States. This was positive news for those hoping it would help make a dent in California’s ongoing drought. But those who watched or experienced the Butte Fire rip through our area feared what the potential deluges could bring down from the blackened hillsides.
While doom and gloom and the potential for mud and debris flows resulting from massive predicted storms filled the media, on came some weather. It rained, it snowed, and early on, the storms yielded after providing some much-needed moisture in a much more gentle manner than feared. These smaller storms helped plants begin sprouting where they are able, and may be helping to minimize the extent to which soil moves from burned hillsides into waterways in this first winter after the fire.
Heavy weather events after a burn can cause great damage to watershed health. Where vegetation once held hillsides in place, landslides caused by saturation of the soil can cause significant harm to property, human life, and ecosystems alike. Particularly after heavier rains, soil newly exposed after duff, protective leaf layers, or roots below ground are burned away can move in large flows or slowly and evenly down a sloped bank. Each bank drains to some form of stream course, be it an ephemeral stream that flows briefly in direct response to precipitation or a perennial stream like the Mokelumne or Calaveras Rivers, which flow year round.
Regardless of how it moves downhill, with precipitation, sediment eventually reaches the rivers. Sediment loads in streams can damage aquatic habitat, food webs, and fish spawning grounds, and can directly kill fish. California Department of Fish and Wildlife Fisheries Biologist Ben Ewing said, “Increased sediment loads can eliminate suitable spawning grounds and can destroy redds (egg nests in the gravel) by suffocating them.”
Sediment movement in rivers, however, is a natural process, and it is normal for Sierra rivers to turn turbid and brown when high spring flows from annual snowmelt carry sediment downstream. This is how California’s Central Valley was formed and why it has such nutrient-rich soil. However, these days most of that sediment is captured behind large dams on our Sierra rivers.
Most aquatic organisms have adapted to these sediment-carrying natural events of peak runoff. Typically species have reproductive cycles evolved to not coincide with these events, and due to the greater volumes of water in the rivers swollen with spring snowmelt, many species are able to find refuges to wait out the flush of the high water and transported sediment. However, when large amounts of ash and soil are transported downstream during non peak flow times of year, the result can significantly harm aquatic species and ecosystems.
Imagine yourself as a fish, filtering water through your gills to pull out oxygen, trying to find a nice spot to lay your eggs on the river bottom. When the concentration of sediment in the water is higher and moving in the stream during an unusual time of year, breathing becomes stressful and ash can clog your gills.
In the Mokelumne River, kokanee salmon living in Pardee Reservoir, and the introduced brown trout, both popular sportfishing species, reproduce in the fall to early winter when they attempt to move up-stream to find suitable habitat to lay their eggs. When these fish are producing and carrying eggs upstream, they are already using a great deal of their energy—both from their journey upstream and from the energy costs associated with growing eggs. Added stress from hindered breathing and the increased difficulties of finding suitable habitat or simply moving upstream in sediment-laden water can easily tip the scale away from successful reproduction or survival.
It’s not only fish that face harm. Depending on the duration, amount, and characteristics of ash material entering a system, macroinvertebrate populations (the small bugs that fish and other small critters eat) can also suffer dramatic reductions. When this part of the food chain is disturbed, there can be longer-term effects to each link of the food chain above them.
In addition, sediment that ends up in streams often carries other pollutants or nutrients that can bind to sediment, contributing to additional stress for aquatic life. Ash and sediment decrease water quality and can even change water chemistry over shorter time periods. Elevated levels of phosphorus or nitrogen can over-stimulate growth of aquatic vegetation, leading to depletion of oxygen levels in the water that can further harm fish.
The East Bay Municipal Utility District and other water purveyors are testing water in the Mokelumne and Calaveras Rivers to determine what, if any, changes they need to make in their water-treatment methods and systems as a result of the fire. Most recently, the Calaveras County Water District announced that it will need new water filtering equipment for its Jenny Lind Water Treatment Plant due to the ash and sediment flowing in the Calaveras River.
The long-term water-quality effects of the Butte Fire won’t be known for some time. But in the short run, the sediment and ash do pose potential challenges for people and aquatic life.