There is much to write about black cottonwood, including: how it's a “hot plant in biology” because its genome has been sequenced; its ethnobotanical uses; whether it should instead be correctly known as Populus balsamifera subsp. trichocarpa; and who its closest relatives are (see Hamzeh, M and Dayanandan, S. 2004. Phylogeny of Populus (Salicaceae) based on nucleotide sequences of chloroplast TRNT-TRNF region and nuclear rDNA. Am. J. Bot. 91:1398-1408. – institution or subscription access required to read).
Instead, though, I'm going to write a bit about stream ecology, since that is what I had in mind when I took this photograph. I look at this image and I “see” grizzlies and orcas. Why? Organic input into streams from the surrounding watershed (such as these leaves) supply nutrients and energy to either detritus-eating invertebrates or aquatic fungi and bacteria (which are in turn ingested by invertebrates). Small predatory fish and large predatory invertebrates eat the detritivores, and are in turn eaten by salmon and other fish. From salmon, it's only one more step to grizzly bears and orca whales. I'm simplifying a little, because there are other foundations in stream food webs, (e.g., photosynthetic algae which then feed invertebrate herbivores which then feed...). Still, one study has estimated that imported detritus supplies over 99% of the energy input in some streams where the headwaters are heavily shaded (see: Fisher, SG and GE Likens. 1977. Energy flow in Bear Brook, New Hampshire: An integrative approach to stream ecosystem metabolism. Ecol. Monogr. 43:421-439.).
In addition to supplying energy and nutrients, plants play other roles in stream ecology. As examples, they are important in regulating stream temperature (overhanging branches provide shade and accompanying temperature gradients), mitigating heavy rainfall by moderating the inflow of water (i.e., ensuring that inflow from rain occurs over a longer period of time instead of heavy bursts that can cause mud slides or flooding), and, of course, providing habitat for forest denizens that rely on the watercourse for food and drink.
Photography resource link: Counting Triangles, an article by Michael Reichmann of The Luminous Landscape. I picked this article today because I only noticed after processing this photograph that the dry area of the rock in the upper left repeats the shape of the leaves in the spiral pattern (as does the rock itself).