Monday, August 12, 2019

A River Ran Through It

Apologies to Norman Maclean for the play on words in the post title. A venture up the Skagit River valley took me across a swampy forest with occasional stream channels. This venture and a little historic research demonstrates the dynamic nature of the Skagit River.   

These channels will fill with water during flood events on the river. The soils in this area are silts and fine sand suggesting that water flow through this forest, when it does happen, is slow. Hence the channels are slowly becoming filled in by overflow river deposits.

These channels are located in an area where the main channel formerly flowed. The 1881 survey map shows the river formed and island in the river.

An island was still present in 1915.

By 1937 the island was no more but the channel was still evident.

By today the channels are filled in with only a slight appearance in tree type indicating the old channel, but at the far end the old channel remains as a lake as it has not filled as much with sediment.

Friday, August 9, 2019

Slide Complex at Mesa del Oro in the Rio Grande Rift

One of my ventures afforded a view of the Rio Grande Rift in New Mexico.

Rio Grande south of Albuquerque

 J Fleck writes about water in the southwest and the Rio Grande water flows. Viewed from above the contrast of the river and irrigated land is sharp.

Mesa del Oro with large landslide complexes on the west side of the mesa 

Mesa del Oro is a basalt mesa made up of lava flows that have come up along the Rio Grande Rift zone. The mesa also has a thick travertine deposit (Priewisch, Crossey and Karlstrom, 2013). The travertine was deposited from springs with volcanic field associated CO2-rich groundwater.   

Basalt lava flows can break into large landslides due to the weak zones of soil soil between the basalt flows and the vertical fractures that develop in the flows upon cooling. It is hard to have a good sense of scale from 30,000 feet. The mesa is a about 800 feet high. The slide complexes from the top edge of the mesa to outer edge of the slide complexes are nearly 2 miles. I suspect that the slide complex has slowly developed as the top edge of the mesa breaks and collapses onto the slide surface the failed basalt blocks have slid and spread towards the west away from the mesa behaving somewhat like a very slow glacier.      

Thursday, August 8, 2019

The Cascade Range Defeats the Stratus

I have been traveling a fair bit. This has allowed me to have more of an opportunity to enjoy hot weather. Northwest Washington rarely reaches 90. My latest venture to hotter climates afforded a nice view of Mount Saint Helens.

Mount Saint Helens rising above the low stratus
The stratus forms in the cool air coming over the Pacific

On this day the stratus layer evaporates as it crests the Cascade Range
A fairly typical event in Washington summer weather