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  • Heidi Cline

Geology, Hot springs of the Middle Fork Salmon River

Updated: Feb 1, 2018




Hot springs dot the banks of the first fifty miles of the Middle Fork of the Salmon. These hot springs are a great way to relax and let all of your cares float down river as you revitalize your mind, body, and spirit.


Let’s take a closer look at what creates these wonderful and treasured oases.


There are two main types of heat sources for natural hot springs. The kind people think of most commonly is related to volcanoes and magma, and the less known but equally as common is related to pressure and increasing temperature under the earth's surface.


Types of Heat Sources for Hot Springs


Because they are the most exciting type, let's first look at the hot springs that are related to volcanoes. In area where volcanoes exist or used to exist, water from rain and snowmelt peculates or seeps into the ground and comes into contact with a body of magma, or heated rocks surrounding the magma body. This water becomes superheated to over 390℉ (200℃). This superheated water is less dense than the cold water seeping down into the ground - and thus lighter - so it is forced back to the surface through tiny cracks in the surrounding rocks. At the surface this superheated water forms geysers and boiling hot springs. Most hot springs related to this type of heat source are too hot for us to sit in, but we do enjoy their beautiful colors and geysers. A great example of these types of hot spring is at Yellowstone National Park. Think of “Old Faithful”.


The other heat source for hot springs at first might not seem as existing, but is actually a really “hot” topic. In this type of hot spring, water from rain and snowmelt percolate into the ground, and are slowly heated as they go deeper into the earth's crust. The temperature of the water increases due to pressure of the surrounding rock layers. Place your hands together and push, do you feel the heat created from you simply pushing your hands together? Now multiply this effect and you can begin to see how the pressure of rocks pushing together can create heat. A pressure cooker is another great example of how heat increases with pressure. This water heats up at a rate of about 2℉ (1℃) for every 100 feet it goes down; going as deep as 10,000 feet. That means the water can heat up to over 200℉ (93℃), around the boiling temperature for water. Once the water is heated it becomes less dense than the surrounding waters and finds its way back to the surface through small crack in the rocks, just like in the first example.


It is this second type of heat source that creates the hot springs on the Middle Fork of the Salmon River.


Why are some Hot Springs Stinky?


As water from the surface slowly makes it way deeper into the earth it not only heats up but it also dissolves minerals, and picks up bacteria from the surrounding rock. The minerals that the water picks up are dependant upon what type of rock the water is seeping through. It is very common for the water to become enriched with many different minerals. Common minerals that the water picks up are lithium, iron, calcium, and sulfur. The rotten egg smell associated with many hot springs comes from sulfur. However sulfur by itself is not the reason for that rotten egg smell.

Some of the bacteria that the water has picked up consumes that dissolved sulfur, and uses it as an energy source. As a by product these sulfur consuming bacteria chemically change the sulfur into hydrogen sulfide. It is the hydrogen sulfide that gives many hot springs their rotten egg smell.


Well fortunately not all hot springs are created equal and not all hot springs have that rotten egg smell. Even more amazingly, the hot springs we encounter on the Middle Fork of the Salmon do not have this rotten egg smell. You will be able to enjoy the healing abilities of the Middle Fork hot springs mineral waters without smelling like an egg!


Names and Milemarks of the Middle Fork Hot Springs


Trail Flat hot spring at mile 7.3

Sheepeater hot springs at mile 13.3

Sunflower hot springs at mile 33.6

Whitie Cox hot springs at mile 46.8

Loon Creek hot springs at mile 50.1

Hospital Bar hot springs at mile 52.9

Check back to learn more as we are always expanding our blogs about geology, biology and the magic of the Middle Fork. Thanks for taking the time to understand just a little bit more about this amazing wilderness, and we invite you to “Come adventure with Us”.


Further Reading

https://www.rei.com/product/862026/complete-guide-to-idaho-hot-springs-2nd-edition

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