By Dave Tucker
On July 16, 2011, twenty people joined the second MBVRC fundraiser field trip of the year to see Mount Baker lahars and other deposits in the Middle Fork Nooksack River valley on July 16. It was, of course, raining. The group consisted of a delightful mix of professional geologists and hydrologists, a couple of archaeologists, and several ‘citizen geologists’ who simply love to learn about Baker volcanism, and geology in general.
We took our two big white rental vans (well, they were only white until we got to the gravel road) and drove up to the end of the Middle Fork. We walked the short trail out to the gravel bars to see the moraine of a valley glacier, capped by the ~6500-year-old Middle Fork lahar, the largest known to have come off of Mount Baker. We also looked at the terraces and a garage-sized boulder left behind by the June, 1927 Deming Glacier jökulhlaup (‘glacial outburst flood), and impressive boulder and gravel deposits left by the January 2009 floods generated by a pineapple express rain-on-snow event. A favorite was looking for, and finding, pebble- and cobble-sized fragments of white “Shermanite”, the hydrothermally altered rock found at Sherman Crater and carried down to this river bank by lahars and floods.
We then walked down to the City of Bellingham’s diversion project down the river, to discuss the potential impacts of a future lahar on the weir, intake tunnel, and pipeline that diverts water from the river to Lake Whatcom, the municipal water supply. We were fortunate to have John Rose (WA DOE) on the trip who knew quite a bit about this project and the potential for a serious water-supply problem.
Further down the river, we laid hands on an exposure of the Ridley Creek lahar, which lies directly on top of the Middle Fork lahar and must have followed it very closely in time. And by then, it stopped raining! Everyone got muddy hands examining the fine clay matrix of the lahar. There was good discussion about the interpretation of the exposure, and lots of speculation about what the river bed must have looked like after it was choked with debris
from the two lahars. There is a lot of alluvium above the Ridley Creek lahar, so the river must have had to do a lot of work to get itself back into a channel and in more equilibrium.
Our final stop was way out in the county lowlands, between Nugent’s Corner and Everson. Here we looked at the barely flowing puddle of the Sumas River, and compared it with the much larger Nooksack. The Sumas is a classic underfit stream, much smaller than the valley it flows in. The Sumas drains northward into the Fraser River north of Abbotsford, while the Nooksack continues west from here through Whatcom County to Bellingham Bay. The drainage divide between the two river systems is essentially the top of the flood levees along the Nooksack- when this river floods, it often send water
north in the Sumas River. The Sumas flows in meanders that are much too large to have been made by such a small stream. It is evident that at some time in the past, the Nooksack flowed where the Sumas does today. good geologic evidence for this is the presence of Mount Baker andesite blocks in buried alluvium across the border in the Sumas drainage. Did the large lahars, Middle Fork and then Ridley Creek, plug up the Nooksack out in the flats and divert the river westward into an abandoned glacial outwash channel? The jury is out on this, but there was good discussion on the potential effects of future lahars. Could they impact the Nooksack similarly, and again divert it to the north? Would Everson, Lynden, and Ferndale cease to be river towns?
The sun came out as we drove back into Bellingham after 8 hours out in the field. The income from the trip will be used to continue MBVRC operations, and to fund future volcano research at Mount Baker.