Narrowleaf Cottonwood

(Populus angustifolia)



Leaves: narrowest of the cottonwoods, lanceolate

Tree: often multi-stemmed along streams

Seeds: tufts or "cotton" in June, or early summer


Floods and Mud

What does a cottonwood seed need to germinate? A combination of luck and evolution. It's not a coincidence that you see Cottonwood "snow" in June. That is the time of year when the stream size is starting to go down, but the stream bank is still muddy. A Cottonwood seed requires wet mud to germinate, but can't survive in standing water. If a seed lands in City Creek but is quickly washed to a muddy shore, chances are it will germinate. But if the mud dries out too quickly, or the stream had another flood pulse, it's curtains for that seed.

(More information on water quality)

That's One Patient Tree

While that all sounds fairly simple -muddy banks aren't all that rare- the process is actually a little more complex. Floods aren't on an exact schedule, and seed release can become a hit-and-miss affair. It's really only about once every hundred years that the stars smile in the heavens, everything aligns and the cottonwood seedlings take root. One hundred years- the life span of an average cottonwood tree and the average flood cycle.

Oh Dam

What happens if there are no flood cycles to facilitate Cottonwood seed germination? Well, look along the banks of any river in the Southwest where Cottonwoods used to thrive. When you take the nature trail on City Creek, you'll notice a lot of the old trees are dying. While there are many factors that could be limiting Cottonwood reproduction, a major one is water diversion, and creation of man made dams. With the introduction of dams came the destruction of many natural flooding processes that riparian systems have evolved with. Dam give convenient flood control but threatened the riparian system's ability to grow and replace itself.

Flood Control Au Naturale

Perhaps a small-scale solution to flood control on bodies of water such as City Creek would be to let nature take its course. Really. The roots of riparian trees (Cottonwoods, Water Birch, Peachleaf Willow) stabilize the streambed and so erosion in minimal when flood pulses occur. Riparian trees are shallow rooted and their roots grow in the mud, through the streambed and even into the water. Natural damming -via beavers- also prevents erosion and traps silt. The worst thing to do for flood control is so remove the native trees from the streambed.

Riparian Trees Box Elder River Birch Human Impact Homepage