In a previous post I described the main findings from a recent paper I had accepted in the journal Water Resources Research. The study looked at the movement of pieces of large wood by using numbered aluminium tags and repeatedly surveying their locations. Out of this study we are able to make some recommendations about how large pieces of wood would need to be in order to be sure they are not going to move. In addition we can highlight the sort of pieces of wood, and conditions, under which there may be high levels of movement.
Logjams in a river have been suggested as important trapping sites for all manner of material moving down the river from leaf packs to large wood. The theory being that the structures act as grates or sieves, letting water pass through them but retaining organic matter (and occasionally all manner of inorganic matter such as car tyres!). By tracking individual pieces of wood over multiple surveys I was able to identify how pieces of wood move into/through/past logjams over time and found some surprising results.
I found that although the amount of large wood in a stream in the general area of a logjam (or indeed in a given river reach) remained fairly constant there was actually a fairly high turnover of individual pieces of wood. What this shows is that although logjams can persist in the same location for a long time and although they do indeed act as effective trapping sites for mobile wood they don’t necessarily retain, or ‘lock’ it in place over multiple years. The below figure from the paper shows a logjam photographed in different years, the tags allow us to identify logs which have move away and new ones that have been trapped. What is fascinating from my point of view is how similar the two photos look. The photos are documenting evolution of the logjam over 3 surveys. The first survey is not shown, it just acts as a reference point. Photo A then highlights newly trapped pieces in the logjam at the second survey (hashed lines) and logs which subsequently move away before the third survey (solid lines). Photo B shows logs which have been newly trapped in the logjam between the second and third surveys (solid lines).
The key implication of this finding is that the trapping potential of logjams is temporary. Their use in river management would need to be along extended sections of channel where it could be anticipated that individual pieces of wood would slowly move down the reach, ‘hopping’ from one logjam to the next as logjams are “blown out” by big floods before component pieces are trapped and racked into the next logjam downstream. It also highlights the importance of very large pieces of wood as anchoring pieces for logjams (in the case of the field photos a big fallen tree bridging both banks). Where logjams can be expected to be broken up in floods there needs to be a focal point to trap mobile wood and reform the jam.
We do not have nearly enough data to say what sorts of wood are more or less likely to be trapped and retained by a given jam, however the data we do have leads me to suspect that more complex pieces (i.e. with branches, rather than straight stems) are more readily retained in a logjam once trapped.
Dixon, S.J., Sear, D.A. (2014) The influence of geomorphology on large wood dynamics in a low-gradient headwater stream. Water Resources Research, DOI: 10.1002/2014WR015947