As I've written several times previously, each November chum salmon spawn in a small ephemeral stream not more than a quarter mile from our front door. Beatty Creek it's called, and I have loved it since the first time I laid eyes on it. This creek differs in almost every respect from Fanno Creek, particularly with regard to its bottom. The waters of Beatty Creek flow over deep beds of well sorted gravel; those of Fanno Creek flow over a bottom that is almost entirely composed of an exceedingly fine mud.
Ironically, these creeks share a common ancestor: the Fraser Glaciation that invaded Washington State during the last glacial maximum. Fanno Creek's geological history is tied to the Purcell Lobe of that glaciation and its relationship to the great Missoula Floods that took place 16-12,000 years ago. The creek lies in the Tualatin Basin, a deep bowl of land in northwest Oregon. The Missoula Floods visited this basin at least once, leaving behind vast quantities of debris, including millions of tons of loess. The muddy bottom of Fanno Creek is directly attributable to that great abundance of that material.
Beatty Creek, on the other hand, owes its salmon-friendly, gravely bottom to the Puget Lobe of the same glaciation. As that monstrous body of ice plowed through the Puget Sound, it carried with it enormous quantities of mineral-rich rocks it had literally ripped out of the mountains of British Columbia and Washington. When the glacier retreated, around 12,500 years ago, it left behind great piles of rocky debris, which in turn were washed and worn by glacial meltwater and all the other agents of the weathering process. The deep glacial drift that covers the lowlands of the Puget Sound are one of the artifacts of that weathering process.