Long ago when my knees, ankles, and the rest of my body were in much better shape than they are now I used to jog along the Des Moines Creek Trail to the Water Treatment Plant and back, about three miles or so two or three times a week. When the weather was good in the mornings I often had the entire trail to myself. Occasionally a plane landing at Sea Tac Airport would fly over and shatter the sounds of the birds singing in the trees and the gentle rumble of Des Moines Creek, but otherwise it was almost always a very peaceful place for a short jog.
It is mostly flat and a straight well paved trail all the way to the Water Treatment plant, so there is no chance of getting lost along the way. Even on warm Spring and Summer days a nice wall of trees on both sides of the trail keep it in the shade for most of the way.
When I jogged along the Des Moines Creek in the Spring and Summer the water levels were always pretty low, but I would often wonder if any fish spawned in it in the Fall or Winter.
The answer is, yes, sort of. I believe there is a project underway to restore the spawning areas for the Chinook, Coho, Chum Salmon, Steelhead and Sea Run Cutthroats that have all traditionally spawned in Des Moines Creek in the past.
I don't think Des Moines Creek is open for fishing itself, but you can fish from the beach near the mouth of the creek. Though be sure to check the current regulations to make sure when and where exactly it is open and what species you can legal catch. Remember, Sea Run Cutthroats are always catch and release only as are any non-hatchery Salmon or Steelhead. Again, you will need to check the regualtions, but I believe the Puget Sound in Des Moines is barbless hooks only.
Though I used to only jog as far as the gates to the Water Treament Plant you can continue past it and hike all the way down to beach and for about another mile along the beach assuming the tide is low enough. If you are feeling really energetic you could even hike up to Marine View Park once you reached the wooden staircase that leads to the trail from the beach.
I wrote about fly fishing for Sea Run Cutthroats along the beach at Marine View Park, but I think this year, being an odd year (2021) Pink Salmon from Mid-July through maybe early September might be possible as well. During high tides and low light conditions Pink Salmon will often come into very shallow water. I have caught three to five pound Pink Salmon from a beach near Bellingham in two feet of water during a good evening high tide. As for Chinook, Coho, and Chum Salmon, I'm not sure if any still return to spawn in Des Moines Creek or not. Hopefully if the restoration to the spawning areas are done maybe they will in the future. But that said, there will still be plenty of Salmon making their way north to spawn in the rivers during the Fall and early Winter. Chinooks, as far as I know don't come in close to shore usually, but Coho and Chum Salmon do. Both species will readily hit a fly if you get it in front of them. Coho like flies skimmed across the surface and will smash them hard if you can get the fly to them. Streamers in various colors (Pink and White for Pink Salmon) work best as well as shrimp patterns.