Why is Lake Chelan So Deep?

Lake Chelan is one of the most popular summer vacation hotspots in the state of Washington.

Lake Chelan is a 50-mile-long lake, that is an average of 1 mile wide.

It is the 3rd deepest lake in the United States at 1,459 feet. Only Crater Lake (1,949 feet) and Lake Tahoe (1,645 feet) are deeper.

Why is Lake Chelan so Deep?

Is it because of the dam they set up in Chelan? Water levels did rise 20 feet - after the dam was set up in the 1920s. 

The actual reason for the extreme depth of Lake Chelan - is because it was dredged or carved out by ice.

Ice from a giant Alpine glacier? 

Alpine glaciers are ribbons of ice that flow down mountain valleys. Alpine glaciers pushed in, stayed for a while, and then formed lakes upon their retreat in the Cascade Mountain region. Examples of lakes from alpine glacier ribbons include Lake Keechelus (near Snoqualmie Pass) and Lake Wenatchee. 

For many years, geologists believed that alpine glaciers created Lake Chelan - until very recently. 

Where did the ice come from that carved Lake Chelan so extremely deep - deep enough where its extreme depth is below sea level? 

A continental ice sheet that came from Canada. Geologists call it the "Canada Ice Sheet."

This is the same ice sheet covering most of the Puget Sound region - in ice as much as 3,000 feet deep. This very same ice sheet covered Lake Chelan with 2,000 feet of ice. The Canada Ice Sheet also created Lake Pend Oreille in Idaho.

So how exactly did Lake Chelan get carved up by the Canada Ice Sheet, if the area is mostly protected by the North Cascades?

A geologist named Jon Riedel has evidence - that Rainy Pass - a low point in the North Cascades allowed an arm of the Canada Ice Sheet to flow over Stehekin and into the valley - now known as Lake Chelan. The ice flowing from the Canada Ice Sheet and up and over modern-day Lake Chelan was an astounding 5,000 feet deep. This magnificent arm was not the only piece of ice responsible for carving Lake Chelan.

It was met head-on by another arm of the Canada Ice Sheet, The “Okanogan Ice Sheet” that flowed from the East and up the lake. 

'These two independent arms of the continental ice sheet met up head-on over Lake Chelan and had a digging contest essentially. Who could dig deeper, to make this hole that's below sea level.”' -Nick Zenter

Where is Lake Chelan the deepest? 

The deepest part of Lake Chelan (1,459 feet)  is just past the “Manson Bend” in the lake.

And WHY is it deep there?

The bed of Lake Chelan’s northern end is made of Gneiss. The bed of the Southern portion of Lake Chelan is made of Migmatite. Lake Chelan’s bed - just past the bend - is made of a softer material called shist. 

Just past the "Manson Bend" is where the two arms of the Canada Ice Sheet butted up against each other and dug deep down into the softer rock. 

'Schist is rich in flaky and soft sheet silicate minerals which makes it structurally weaker than gneiss' -Sandatlas.org

Nick Zentner goes in-depth to explain the geology of Lake Chelan - and even explores the great Entiat earthquake of 1872.

There is evidence that a major earthquake seems to occur near Entiat - every 200 years.

This means we are slightly less than 50 years from yet another major earthquake in North Central Washington.

INFO: Nick Zentner, Central Washington University Geology professor