Thalweg Surveys

Nerd Terrarium warning: Some terminology and concepts may be considered nerdy.

My first thalweg survey was in a Forest and Range Hydrology class at Cal Poly San Luis Obispo. We were surveying San Luis Creek downstream of downtown San Luis Obispo. I remember we reached a big, deep pool, and as a taller person, I got to go into it with the survey rod because I could reach the deeper parts while still standing. I remember climbing onto a big log and reaching out into the pool to place the rod in what I thought was the deepest point.

And that is what a thalweg survey is. The thalweg is the deepest part of a channel. If you are in a boat and not wanting to run aground, you would typically follow the thalweg. If you were walking up or down a stream, unless you wanted to get really wet, you would typically avoid the thalweg. A survey of the thalweg is done in order to produce a profile view of the creek--from which you can see the slope and pools and riffles.

On Sunday I walked downstream. It was a hot day, but I wasn't prepared to go in too deeply or do too much climbing over debris because I was carrying my sleeping son in a carrier on my chest. Nevertheless, I got farther than the debris-jam and pool that stopped me last year. Peak flows during the winter cleared the jam and deposited sediment in the pool.


I reached a location where a landslide during the winter had carried some upright trees down the left-bank slope and into the middle of the creek. Flow had scoured around the new island, but most of the rocks on the left side of the island were sharp-edged colluvium--deposited by gravity, and not the rounded rocks deposited by flowing water.

Just below this interesting spot was a debris jam in a pool that was covered in wispy white mucky foamy traces. Yuck--I didn't want to step in there because I couldn't see the bottom and the floating muck wasn't appealing. But after taking a closer look, I could see it was shallow and I stepped in, reaching the debris in the center. Balanced on the debris, I evaluated my options. I could walk on a large floating log and hope it holds me, but once I got to the end I would have to hope the water was shallow enough to step in. It looked deep. Or, I could climb over the debris to the other side, but still have to cross the deep pool, and I'd have to get up close and personal with some poison oak. Neither was a good option for today, so I turned around.

On my way back upstream, I noticed a very large planted poplar that was at least five feet in diameter. It was as large as many of the largest redwoods in the valley and must have dated back at least a hundred years. It was majestic.

In another spot, a large frog jumped into the creek as I approached the bank it was sunning itself on and swam as fast as a fish away. Within the next fifty feet three more frogs--all much smaller--jumped in as I came too close.

On Monday I walked upstream. It was an 84-degree day. I found the pool that stopped me last year to still be impassable without swimming. There is rip rap at a lot of properties, channelizing the creek and maintaining its current configuration. Someone (long ago?) had hung cans in the trees and had shot at them and at other debris in the banks.

I found a cluster of several golf balls downstream of the Larsen Creek confluence. Golf balls travel like bedload--they roll along the bottom of the stream. After one storm last winter, there were bucketfuls of golf balls downstream of the Larsen Creek culvert under Sir Francis Drake Blvd. I wondered if there is a TMDL (Total Maximum Daily Load) for golf balls downstream of golf courses. I wanted to go down and fill up a 5-gallon bucket half-full with them. But I didn't make it in time before the next storm covered them with sediment or moved them farther downstream. Each subsequent storm caused more of them to disappear until now they are all gone.

There are reaches with a lot of bedrock and reaches with a lot of sediment in alternating point bars. The bars that are out of the water can be traced through the riffles under water and seen as a single continuous feature, with the thalweg crossing back and forth, creating pools alternating on either side of the bar. To see this, just focus on the sediment and not on the low-flow location of the water. You can see this in many rivers and streams. The sediment moves both longitudinally and laterally as the pools are scoured and the bars slowly migrate.

When the property was subdivided for the house we are renting, the creek ran right through where the house now stands. An old assessor's parcel map showing the creek where our house is raises the question--did the creek migrate on its own, and then the house was built, or did the creek get moved so the house could be built? I think the latter must have happened, because otherwise the parcel where our house stands would have been unbuildable--who would have subdivided it that way unless they planned to move the creek over to the edge? It appears that the house is built on fill, and the downcutting and bank erosion along the creek in its current channel seems relatively recent, but there are few other clues to this mystery. One clue seems to be that the sediment-bar-dominated channel ends about where the channel would have been moved, and it becomes more bedrock-dominated. It is hard to tell how else the character of the channel changes, if at all, but a LiDAR survey would probably show a lot of interesting patterns that otherwise are obscured as one stands in the channel, dwarfed by the scale of the geomorphological mystery.