Every winter high surf and rivers swelled by drenching storms combine with high tides to replenish the wrack line. It’s an annual cycle of renewal, muted only when winter storms are mild. Recent winters on my home beaches have seen few storms, low surf, and little replenishment of the wrack line. That changed this winter. Compare the photos below to see what replenishment of the wrack line looks like.
What’s the fate of all that wrack line material? Intuition and experience tells me beachcombers take home the useful and transportable timbers and lumber. The smaller material is degraded by weather and the erosive forces of wind-driven sand. Beach hoppers, Megalorchestia columbiana and M. californiana, and the backshore isopod, Alloniscus, play their parts too. Wrack line material, mostly with terrestrial origins, provides transient homes and nutrients for the plants and animals of the backshore. It’s a connection between upstream terrestrial ecosystems and the sandy beaches.
This discussion has been about material deposited by high tides and surf above the main beach on a shelf at the base of the foredune. When you’re out on a sandy beach give a look around for the shelf and what you’ll find is, no shelf, no wrack line. Material deposited on the main beach will soon wash away. Or will it?
The beach got a deposit of big wood on January 23, 2016. How much staying power will these big stumps and logs have?
Examining the new stumps I noticed several were carrying rocks trapped amongst the tangle of roots at their base. You can see a big one in the photo at the top left of the gallery above (click on the photo for a full-size view), and here’s a closer look at some smaller rocks I found in the same stump.
Where did this stump and the rocks it carries come from? How many of the strange rocks we see on the beach arrived in the root masses of ancient stumps? This is a beach connection with upriver forested ecosystems we hardly ever think about. It has similarities to bull kelp transport of rocks from subtidal ecosystems onto the beach. I wrote a little about that in Bull Kelp Drift: A Subtidal-to-Surf Zone Connection.
Beachcombers quickly take advantage of the replenishing effect of winter storms. They are out as soon as the storms pass, and it doesn’t take long for the best treasures to get carted away. On the human-made side of the wrack line equation I’m always looking for floats, bottles, and other mysteries.
To see these and more human-made items, plus other things I saw in the wrack line on January 23, 2016 click here.
The northern hemisphere spring is on the way. Soon big storms will give way to mild conditions and small waves. The wrack line will dwindle to drift logs on backshore shelves. Until then, there’s still time to comb the wrack for treasures. Or, you can explore my wrack line galleries from over the years here.