Record rain to begin the New Year - 2.26" fell at the Olympia airport on the 2nd of January, 2021, the day I started working on this post. Our backyard rain gauge showed more than 6" for the week (12/28/20 - 01/02/2021). I plot rain totals sporadically, and by the time I got to the gauge Saturday, it had long since been filled to overflowing. I decided to visit Beatty Creek and see what it was up to.
I headed for the place I've come to call The Bend. It's a great place to study basic stream dynamics. All but the creek's uppermost reaches flow over land once entirely covered in the deep glacial till deposits left behind by the comings and goings of the Puget Lobe of the Fraser Glaciation. The till lies upon large deposits of weathered basalts mostly visible only in the uppermost portions of the watershed. The creek’s headwaters are not far from the Puget Lobe’s terminal moraine, which means it lies in one of the first regions of Puget Sound to be reclaimed by vegetation after the last Ice Age. That was somewhere around 12,000 to 15,000 BPA (source references dependent). The area’s lowland soils consist of a shallow top layer of sands, gravel and decaying detritus lying over a deeper layer of semi-consolidated sands and gravels which covers a deep and highly compacted layer of gravel and sand – “hard pan” it’s called and for good reason. Over the millennia the creek has meandered across these lowlands more or less at will. Maps based on older surveys suggest the reaches I study have migrated over a hundred yards east of their current location.
Stream flow reaches the Bend after passing through a relatively long and straight reach. For the area I'm discussing, total elevation change is not extreme - about an 80 foot drop over a half channel-mile of terrain. But the flow is still packed with a substantial portion of the energy it gained as it tumbled out of its headwaters area 3 miles and 1200 feet higher in elevation. It's also loaded with suspended solids - rocks, gravels, sands - that add to the abrasive actionof the flow. The weight of the atmosphere and the pull of gravity keeps that powerhouse from literally exploding when it slams into the 90. angle at The Bend; instead, it boils its way through the curve and continues downstream - checked but not broken and regathering power and material as it flows. As tumultuous as the surface looks, the real action is taking place in the water body below – the cutting, grinding and plucking actions taking place along the steeper portions of the bank are fierce and relentless.
Artifacts of this continuous process are abundant in the area. For example, in the photo above, the downed tree stretching left to right across the frame was upright and a good foot from the edge of the channel when I first came to this place in 2014. It's been down and in the channel since the fall of 2016. The Bend itself – its hard-left turn – was not quite so pronounced, and the gravel bar directly behind the upright snag (right-center) was not as well developed.
Flow events at The Bend are also impacted by changes in the channel immediately downstream. The large debris pile shown below (right of center in the photo) has split the stream, creating a good sized bar in the middle. This blockage actually dams the flow enough to raise the water level upstream, which increases the height and breadth of the cutting action taking place in what you might call the stream’s elbow. These kinds of obstructions come and go and are constantly changing shape and characteristics. They are crucial to the continued good health of fast flowing streams such as this one. How long they last can vary from hours to decades, many factors dependent. Right now this formation promotes rather than hinders salmon spawning; but other such piles of debris have totally prevented any fish from this year’s chum run making it to these upstream areas.
Below, a recently fallen tree just a few yards downstream is beginning to trap debris between it’s trunk and the water. I suspect it won’t have much impact on stream flow until it breaks in half, or until the bank supporting one end or the other erode enough to put it directly in the water. That could take several decades.