How stable is your bed?

Coloring tracer rocks

If you have spent much time on a river, there is a good chance you may have stumbled across some river rocks that seem out of place. Maybe you pick one up and realize it has been sprayed with a coat of paint. You may not have thought much of it and tossed it back, or perhaps you took it home. Either way, it might surprise you to learn that such rocks play an important role in studying how sediment moves in relation to changes in river flow.

Gravel mobility or transport is important for salmon because it keeps the gravel oxygenated, removes algal growth, and decreases the amount of cementation that occurs when smaller particles fill the spaces between immobile gravel (Kondolf, 2000). On the other hand, too much gravel movement can decrease salmon spawning habitat by removing gravel from areas that are suitable for spawning and relocating it to deep pools that are unsuitable.

Recovering tracer rocksQuantifying the amount of gravel or sediment transport is a difficult task, but it’s made easier with the help of colored rocks.  Prior to a prescribed change in river flow, we collect stones from the river, dry them, and mark them with a bright color that can easily be spotted on the riverbed. Afterward, we return the rocks to the river and place them every few feet along a transect. We measure each rock and record its size and distance from the riverbank (Wilcock, 1997). After the river has pulsed and returned to stable conditions, technicians return to the location to determine how much gravel movement has occurred due to the variation in flow.

However, every colored rock is not always found. Sometimes they get relocated much farther downstream than anticipated, get buried beneath other gravel, or perhaps have become a recent addition to someone’s landscaping, Nonetheless, these rocks provide valuable information about how much movement occurs among the shifting stones, and what sizes of gravel can be displaced with fluctuating flows.

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