Dredging

Dredging in Petaluma – Historical Background

In 1959 Congress declared the Petaluma tidewater slough to be a river. This designation from slough to river authorized periodic dredging paid for by the Corps of Engineers to remove sediment which accumulates on the bottom of the river, due to runoff from surrounding uplands and tidal deposition from San Francisco Bay. Justification to pay for dredging comes from the commercial tonnage of three river-using companies in Petaluma: Dutra Materials, Jerico Products, and Shamrock Materials. Dredging of the river is normally done about every four years. However, because of funding problems, the dredging scheduled for 2000 was implemented in 2002 and completed in 2003. Removal of this sediment maintains navigability of the river and is important to the economic health of the city.

Dredge Boat

In 1975 an agreement was reached between the City of Petaluma and the California Department of Fish and Game (CDFG) to create a dredge-spoils site at the present location, known today as Shollenberger Park. This agreement stated that the city would establish by 1995 a permanent open-space easement, including a habitat management plan, protecting the 65 acres fronting the river. Also the agreement restricted dumping of the spoils from the river bottom into the eastern part of the site.

In 1985 the city wanted to locate the Petaluma Marina at the site of the “original” Shollenberger County Park. The city and county entered into an agreement whereby the city was to relocate Shollenberger Park to the new dredge-spoils site and open the site to the public as Shollenberger Park. In October 1996 the new Shollenberger Park opened to the public. To create the new park the city built the parking lot, a bathroom, and paved the first half-mile of trail along the northern part of the park to the river.  This is legally all there is to the park property, but most people, including some city employees, consider the entire wetlands area a part of the park.

Dredge Pipe to OutfallAs of fall 2012 dredging has not occurred since 2003, partly because the city has yet to write the management plan it promised to create in 1995, and also because of the current poor economy. Money for dredging comes from the Corps of Engineers via prodding from our local congress person. When dredging is to be done, the city contracts a dredging company and the Corps of Engineers pays the bill. Numerous inspections are required before and after the dredging is completed.

A dredger boat has some unique features. It is designed with two vertical posts that move up-and-down and can be used to anchor the boat at any point in the river. The boat can also pivot around a post so that the working component of the boat can sweep from side-to-side in the river. The working tool resembles a huge potato whip or blender 3-4 feet in Dredge Spoils at Outfalldiameter and many feet long. Its shield, containing several blades, is lowered to the bottom of the river where it chops up the sediments as it slowly sweeps back-and-fourth. A stream of river water from the river surface is constantly added to dilute the water-spoils mix which is pumped through a flexible pipe system up to three miles long and 24 inches in diameter to deliver the sludge to the pond. At this point the sludge is about 10 – 15 percent sediment, and the rest is river water. Birds gather around the dredge outfall because it is very rich in fish and invertebrates which they eat immediately and for many months following. Mammals such as river otters, muskrats, raccoons, skunks, etc. may also join in the feast. The scraps from this feast quickly degrade to detritus, which enriches the water to stimulate plant and invertebrate growth. Dredging is usually done in autumn to protect migratory fishes, but if the pond gets too full some of the water will be skimmed (decanted) off the surface (after the sediment has settled) and released from the pond through one or two water gates where it flows to Adobe Creek, then back to the river.

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