Denny Hill Still Exists

In Which I Explode a Popular Myth

One of the biggest misunderstandings in the story of Seattle is that Denny Hill no longer exists. My job in this newsletter is to show you that it’s still here. You just need to know where to look and why to look there. For those ambitious types, it would help if you were a diver who could descend into Elliott Bay.

Between 1897 and 1930, five regrades lowered Denny Hill by 100 feet, reducing it from a mid-sized hill to its present pancakian topography. For the first three regrades (1897, 1903, 1907), workers primarily dumped the sediments along the waterfront though some ended up around Pine and Olive Streets, carried by a small railroad known as the “Mount Moses, Denny Hill, Central and Westlake Valley Airline.” The fourth and fifth regrades were different.

Seattle P-I, First Avenue Regrade, Aug 8, 1898

For the fourth regrade between 1908 and 1911, engineers decided to dump the hill out past tideline into the deep water of Elliott Bay. The muck would travel via an 875-foot-long tunnel excavated from Bell Street and Second Avenue to Elliott Avenue. The timber-lined tunnel, tall enough for a person to stand in, led to a flume, or sluice, that passed over the tracks and roadway of Railroad Avenue and discharged the hill into the water. By the time the contractor finished the regrade, the flume extended on pilings (up to 125 feet long) for 1,200 feet into Elliott Bay, creating a navigation menace. In addition, the dumped sediments resulted in water muddy enough to drive away fish, according to local anglers.

For the final regrade (1928-1930), engineers tried another approach. A scow would carry the Denny Hill sediment out into Elliott Bay. Designed by naval architect William C. Nickum, the scows were the same top and bottom with two internal tanks, one on each side of the scow, and an open deck that held 400 cubic yards of dirt. A tug towed the full scow out into Elliott Bay, where a crewmember pulled a rope that opened valves, or seacocks, on one side of the boat. Within three minutes, water filled one of the tanks, and the out-of-balance scow flipped over, dumping its load. No longer weighted down by the dirt, the scow rose high enough to drain the internal tank, which took about eight minutes. The tug then returned the scow back to shore, ready for its next load.

Images from Too High and Too Steep

The scows, or the scow workers, did not always perform as planned. In September 1929, one scow smashed its mate causing it to sink. Three weeks later, after both barges (which cost $15,000 each) were repaired, workmen left the seacocks open on one, which caused it to flip and hit its tug, which promptly sank. A week later, the remaining scow flipped while at dock, damaging the dock and scow, both of which were put out of service, forcing the entire regrade project to shut down for several days. Tug drivers also had the problem of the scows disappearing in foggy weather, which caused delays.

In 1988, UW oceanographer Mark Holmes and two other researchers decided to look more closely at the seafloor of Elliott Bay. The earliest map they consulted, an 1875 hydrographic chart, showed that close to shore, the seafloor dipped gently and uniformly. When they looked at a map from 1935, they found that the moderate slope was gone, replaced by a shoal, or linear landform that just didn’t “look right,” says Holmes. Its irregular upper surface was fifty feet underwater with abrupt sides sloped steeply to the west. The center of the mound was about 600 feet offshore between Piers 64 and 69.

USGS Bathymetry. Blue is deeper. Orange is shallow.

Comparing the earlier map with one produced in 1935, Holmes’ team was able to determine that the structure ranged from 10 to 120 feet thick and measured 1,500 feet wide by 2,500 feet long. They estimated its volume at 8.9 million cubic yards. The underwater shoal was unlike any natural feature typically found in Puget Sound, says Holmes. In order to get a better picture of the curious mound, Holmes’ team sailed into Elliott Bay on the research vessel Coriolis, equipped with a bubble pulser, basically an electronic bat except that the pulser’s energy waves can penetrate into rock. The seismic reflection data clearly revealed a hummock primarily made of coarse sand and gravel with flanks of clay and silt. They had found Denny Hill.

If you look at the bathymetry chart above, you can clearly see that Denny Hill still exists though it’s now underwater and now home to marine invertebrates and the like instead of people. So don’t believe everything you hear or read about Seattle…unless it’s from me!

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Amazingly, you can watch an 18 minute video that shows the scow and some of the excavation, as well as provides history on the project. Sorry for the initial ad and propaganda (though it’s sort of charming).

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Tuesday, September 27, 10am - Who’s Watching You. I am leading a 1.5 mile long walk looking at carved and terra cotta figures in downtown Seattle for the Seattle Audubon Society. I think the walk is a rather fun adventure.

Thursday, September 29 - 7pm - Liberty Bay Books - I am crossing Puget Sound to Poulsbo to talk about Homewaters.