Coldwater Lake never ceases to amaze me. It’s a huge, natural lake near the top of a mountain, yet it is younger than me. I always think of lakes as old as the hills, but features on volcanoes sometimes are quite young.
Mount St. Helens Looms Behind Johnson Ridge with Coldwater Lake in Foreground.
Coldwater Lake was a twinkle in Mount St. Helens’ eye on May 18, 1980. The mountain blew it’s top at that time in the now famous eruption. Although the lake did not form immediately, that eruption started the process. The Coldwater Valley did exist then, but the explosion and subsequent lake formation altered it.
Coldwater Lake Beginnings: Eruption, Landslide, then Human Tweaks
The enormous landslide that formed Coldwater Lake originated when Mount St. Helens north flank collapsed, causing an absolutely gigantic debris avalanche.
Before the eruption, Coldwater Creek flowed through the valley. When Mount St. Helens erupted, a landslide slid into the North Fork Toutle River Valley. It blocked the creek. Coldwater Lake slowly formed as the creek couldn’t get past the landslide. In 1981, the Army Corps of Engineers excavated an outlet channel to the Toutle River. This channel alleviated the pressure of the landslide so that Coldwater Lake would not breach the dam (United States Geological Survey [USGS] 1999).
This landslide was a debris avalanche.
Debris Avalanche: A huge mass of quickly flowing material consisting of a wide variety of sizes and shapes of rocks, soil, ice and water.
The landslide from Mount St. Helens’ collapse overtopped Coldwater Ridge and into Coldwater Valley, blocking Coldwater Creek. This formed Coldwater Lake. Coldwater Ridge is now named Johnson Ridge after the geologist who died on that ridge that day.
In 1985, there was an additional tweak to the lake. Its source of water grew. A tunnel funnelling Spirit Lake water into Coldwater Creek was drilled at the time. The diversion stabilized Spirit Lake’s water level. This not only resulted in more water, but created a new landform. On the southeast shore of Coldwater Lake, where the South Fork Coldwater Creek empties into the lake, a delta grew quickly afterwards (USGS 1999).
Coldwater Lake Lateral Moraine
A lateral moraine flanks the NE side of Coldwater Lake. It dates to 22,000 to 11,000 years B.P.
Believe it or not, despite the massive debris avalanche, the damming, the tunneling, and the actual lake formation, an old glacial feature towers over Coldwater Lake. A lateral moraine flanks its northeast side. It’s huge! It’s old too: 22,000 to 11,000 years B.P. (Burns n.d. [but before 2011]).
Coldwater Lake Vegetation
By 1983 and 1984, vegetation grew on the slopes flanking Coldwater Lake.
Vegetation did not take long to recover. It was bare after the 1980 eruption. The Coldwater Lake was within the blowdown zone (defined below). However, a few plants on the landslide survived by ending up on debris avalanche’s surface. The fireweed-pearly community was slightly returning by late 1982. There was still 62% bare ground then. In 1983 and 1984, the slopes around Coldwater Lake were strewn with vegetation, but the flat areas were still barren (Anderson et al. 1985:348-349; United States Forest Service [USFS] 2000).
Blowdown Zone: the area where Mount St. Helens’ 1980 lateral blast knocked down the forest.
Blowdown Zone – the forest blown over by Mount St. Helen’s blast on May 18, 1981.
Coldwater Lake Fish
Water transparency gradually improved within Mount St. Helens Lakes like Coldwater Lake. Algae provided nutrients for life to grow back.
The Washington State Department of Fish and Wildlife stocked Coldwater Lake with rainbow trout during the 1990s. Before then, no fish swam in the lake. Mount St. Helen’s eruptions killed most of the fish swimming in streams, including Coldwater Creek. Water transparency in most lakes in the blowdown zone improved greatly within a few months after the eruption, improving conditions for fish. No additional stocking has occurred due to their self-perpetuation. The rainbow trout have been seen in streams associated with the lake (USFS 2015).
Anderson, Roger Y., Edward Nuhfer, and Walter E. Dean
1985 Sedimentation in a Blast-Zone Lake at Mount St. Helens, Washington – Implications for Varve Formation. Geology 13:348-352. Electronic document, http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1305&context=usgsstaffpub, accessed July 7, 2015.
n.d. (before or including 2011) Field Guide to Mount St. Helens North. Electronic document, http://web.pdx.edu/~ruzickaa/G200/G200%20MSH%20north%20guide.pdf, accessed July 7, 2015. Note: This field guide references two items by Pat Pringle. They’re really good. Check them out!
2015 How Were the Fish Affected By the Eruption?: Frequently Asked Questions. Mount St. Helens. United States Forest Service: Pacific Northwest Research Station. Electronic document, http://www.fs.fed.us/pnw/mtsthelens/faq/q10.shtml, accessed July 7, 2015.
2013 Lakes and Drainages around Mount St. Helens. USGS Volcanic Hazards Program Website. Electronic document, http://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_107.html, accessed July 7, 2015.
1999 Volcano Landslides Can Dam Tributary Valleys. USGS Volcanic Hazards Program Website. Electronic document, http://volcanoes.usgs.gov/Imgs/Jpg/MSH/Coldwater1981_Topinka_caption.html, accessed July 7, 2015.