Pebble Count Conducted at Milk Creek Site

Last month we conducted a pebble count on Milk Creek at the site of the vegetated log matrix that was built last summer. A pebble count is a common method used by river restorationists and geomorphologists to sample particle sizes in a particular stretch (“reach”) of a stream. It involves randomly picking up and measuring the diameter of rocks and cobbles on the stream floor.

The pebble count gives us valuable information about what sediment or gravel Milk Creek is moving through the stream system. Knowing what size gravel is in Milk Creek tells us about levels of velocity, and if the velocity is changing, and whether the reach would be suitable place for spawning salmon.

The first pebble count on Milk Creek was done before constructing the vegetated log matrix, and gave us a baseline for comparing future measurements. So what did we find out? The average or median particle size in this reach of Milk Creek was “coarse gravel” to “very coarse gravel.” It’s the size of rock that salmon look for in order to build their spawning nests (“redds”).

We are keeping a watchful eye on the reach and hope to spot salmon redds here in the near future!


Clackamas SWCD