Since our lives are relatively, so short, we tend to think our common landscape features change little over time. Who would think for example, that the Nooksack River was once a tributary of the Fraser river, or that 10,000 or so years ago almost none of our favorite fishing spots in the Valley and LM existed. They would have been covered by hundreds of feet of seawater. The valley up to Hope was a saltwater sound. Both Harrison and Pitt lakes were not lakes but saltwater inlets of the Salish Sea.
The whole area from the north shore south to Bellingham and east to Agassiz or Hope used to be the Fraser river delta. 12,000 years ago it was covered in Ice. As the >1km thick glaciers retreated, and the Fraser had a
much higher water flow, the glaciers ground down the coast mountains and the Fraser deposited all that sand and gravel at it's mouth, just like what it does at sandheads today. Also as the weight of the glaciers was reduced, the whole area slowly rose out of the sea (post-glacial rebound).