A river with its strict sense of flow is a universally used symbol for the passage of time. Resistance against that is, in contrast, best contemplated by looking at the relentless forth and back of waves along a coast line.
Time is reduced here to repetition, it seems.
The weeds and twigs shown are gone. Their only action then was to write on the water, invisibly, and immediately forgotten.
But there is more. They are defending a territory beyond the water.
They are also witnesses for an esthetic of complexity beyond the untextured and timeless water.
And finally, they did leave their traces, elsewhere, in memory.