The landscape of the western Washington low lands is full of valleys with undersized streams. That is, the existing stream in the valley did not erode the valley. Some past larger water flow carved the valley that is now occupied by a rather smallish stream. Many of these valleys were formed during the late stages of the last glacial period as ice retreated from the western Washington low lands. Large rivers diverted by glacial ice carved deep valleys and then found new routes as the ice retreated and opened up new paths for water flow.
A great example of one of these valleys is just north of Bellingham. It is a rather pretty valley with a mix of mature second growth forest and pastures without too much in the way of rural sprawl so it has maintained its rural character despite its proximity to Bellingham.
View of Squalicum Valley from Van Wyck Road
The above view is actually a section of the valley upstream of Squalicum Creek proper
This valley shows up even on topographic maps as an obvious glacial river channel, but LiDAR imagery makes an even clearer picture.
A river incised down through the landscape when the ice margin was a bit north of Bellingham. Perhaps it was a river of melt water issuing from the margin of the glacier or it may have been a diverted Nooksack River. The Nooksack currently flows from southeast to northwest across the northeast comer of the image. If the river had been present its path would have been blocked by the ice and diverted southward to what is now Bellingham.
Additional channels of similar size can be seen to the north of Squalicum Creek. These too were formed by past larger volumes of water as the glacial margin retreated further to the north. (There has been suggested ideas of readvancing ice as an explanation as well, but that idea has been loosing favor of late).
Just northwest of the name Bellingham on the LiDAR image, the valley fades away and a narrower valley continues to Bellingham Bay (the squarish cut out is an old gravel mine). This area was formerly a delta area and suggests that the large Squalicum Creek valley was formed when local sea level was substantially higher than present as the area had yet to rebound from the mass of glacial ice. I came across some marine fossils very near Squalicum Creek in Bellingham (tube-worms-in-bellingham-drift). As rebound took place Squalicum Creek down cut through the rising terrain.
The comparison of the upper broad deep valley with the lower narrow valley is informative. The existing lower valley carved by Squalicum Creek is dwarfed by the much larger valley it occupies upstream. Squalicum Creek has occupied the lower valley for something on the order of 12,000 years. The upper valley was likely carved over a period of less than 1,000 years and perhaps even less than 100 years.