Bedrock channels are some of the most beautiful places on Earth. They also play an important role in the erosion of the land surface and the transport of material (both organic and inorganic) to the sea. Unfortunately (or fortunately, depending on how you look at it), bedrock channels are often in extremely remote places and have never been monitored or “gauged”. This has caused many studies of bedrock channels to be comprised of intermittent visits during low flow times. Developing of record of how a bedrock channel distributes shear and sediment during floods is extremely useful to understanding how a bedrock channel evolves. The intermittent, “low-flow” monitoring has much in common with observational marine geology. Bedrock streams, like Eagle Falls, are also regulated by many of the same processes that are important in submarine canyons.

Unlike submarine canyons, there are a few bedrock channels that can be accessed without mounting an expensive scientific expedition. Eagle Falls is one such place. It is a bedrock reach of the South Fork Skykomish River, which drains the Cascades east of Seattle. Eagle Falls is only about a one-hour drive from Seattle, adjacent to US Route 2. Therefore observations can be made during unusual events. Changes between low flow and high flow times can be dramatic. The photo below on the left was obtained during the low-flow summertime (26 Sep 2002, discharge ~ 10 m³/s), while the photo on the right was taken during the peak of a winter flood (26 Jan 2003, discharge ~ 1,000 m³/s). For scale, the field of view of each photograph is about 30 m across (and yes, it was very scary taking the picture on the right).

I and my students have monitored the falls for nearly five years. In this time, we have begun to observe the formation of a pothole. A pothole is a depression in the channel that contains sand, gravel and cobbles, which are trapped in the depths of the depression. As water swirls in the hole, the stones gradually erode the sidewalls and base, making the hole deeper and wider. The larger hole captures more material, which increases erosion and so on. This positive feedback, and the deep potholes it produces, appears to be one of the most dominant mechanisms of erosion at Eagle Falls.

The three photographs above show the same area in three successive years (2002, 2003 and 2004). This region is the site of an active pothole just below the bench seen in the two photographs at the top of the page. The pothole appears to the site of gravel/cobble storage, with accumulation of large boulders at times. The largest boulder (weighing approximately 2 tons) seen in the first two frames above was “abrading in place”. That is, the boulder was wearing down in a similar manner to the bedrock substrate around it. In a large flood event in the fall of 2003, the largest abraded boulder was flushed out, leaving only gravel and cobbles, typical of most potholes. With the boulder gone, we intend make a high-resolution survey (millimeter accuracy) of the area each year. The results of the first two surveys are shown here. As can be seen in the plot, substantial erosion of the sidewalls occurred between 2004 and 2005. We hope to relate the pace of erosion with the frequency and magnitude of the floods as measured by a series of nearby gauging stations.

Jeff Parsons’ Research UW-Oceanography