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 Smith River Project > Changes in the Estuary, Lower River, and Lower Tributaries

Chapter 6 -- Changes and Trends in Stream Habitat and Fisheries (continued)

Changes in the Estuary, Lower River, and Lower Tributaries

After the most recent ice age, sea level increased steadily until 3000 to 6000 years ago, and then stabilized within five meters of its present elevation (Burke personal communication 1996). So the Smith River estuary has existed in its present location for only 3000 to 6000 years, which is very recent in geologic time scales.

After sea level stabilized, there was probably occasional upheaval over the centuries. Considering the geologic evidence in other coastal bays of the region, it is likely that the estuary subsided by one or two meters following major earthquakes on nearby faults. This causes sudden inundation and alteration of shoreline vegetation and other features. Also the estuary has probably been hit by tsunamis several times over the last several thousand years.

Early written records of the coastal rivers of the Pacific Northwest describe the original wild rivers. The main river channels were composed of large networks of sloughs, beaver ponds, driftwood dams, swamps, and grass marshes. Flooding was frequent due to beaver dams, accumulated sediment, fallen trees, and living vegetation in the channels.

Early accounts describe an unbelievable abundance of large woody debris in the rivers. For instance, in the Willamette River in Oregon, government crews recorded the number of instream trees that were pulled from the river to allow navigation. Some 5,500 trees were pulled from a fifty-mile stretch of river. The logs had diameters of five to nine feet and were from 90 to 120 feet long. Lower main channels of some rivers were completely impassable due to log jams. An especially large driftwood jam on the Skagit River covered about 120 acres (Sedell et al. 1988).

Possibly the lower Smith River and estuary had similar conditions prior to major modifications. Although the following description of the estuary from 1881 (Bledsoe 1881) probably occurred after some modification, it describes a complex estuary that contrasts with present conditions. "Near [the mouth of Smith's River] are a number of sloughs, branching out from right and left." "Sloughs... branch out from the river on either side, and form a network of safe harbors in which logs could be confined. ...There is very little current in the sloughs even during the highest freshets." "A smooth level beach affords excellent facilities for hauling seines, and ...there are no rapid currents or obstructing drifts."

The estuary and lower river were modified to expedite navigation, transportation, logging, and agriculture. Eventually these modifications included channelizing, diking, removing woody debris, and possibly dredging. Perhaps channels of some lower tributaries were used as log hauling routes. Levees were constructed and riparian forests dwindled. Floods in 1955 and 1964 decreased pool depths in larger streams and the estuary. Overall these changes greatly reduced habitat diversity and instream cover complexity in the lower river and estuary (McCain et al. 1995). In particular, it appears that there has been a great loss of connectivity between Rowdy Creek and the estuary.

In the lower river, it appears that the volume of gravel has increased since European-American settlement. Years ago, gravel mining was advocated to reduce aggradation and thereby increase access for migrating adult salmonids (California DFG 1980). Gravel mining in the lower Smith River has supplied most of the gravel for Del Norte County. Presumably aggradation and gravel mining has altered spawning habitat in the lower river but long-term benefits or impacts on fish are uncertain.

Levees upstream from the "cattle crossing" restrict the lower Smith River from flowing onto its former flood plain. The levees restrict the width of the channel, increase water velocity, and influence bedload transport rates. This may result in transportation of greater quantities of cobbles into the estuary.

The levees also direct the erosive force of peak flows onto coastal sand dunes. This increases the likelihood that the river will erode through the sand dunes to create a new mouth farther to the south. This has happened before. In 1942 and 1966, the Smith River had two outlets to the ocean: the present northern mouth and a southern mouth (USFS aerial photograph, USGS 1966). In 1966, the southern opening to the ocean was located about 0.7 miles (1.1 km) from the northern mouth.

The rate of sand dune formation is usually sufficient to outpace erosion by the river. Along the coast, sand dunes gradually move in a northern direction. This causes the river mouth to gradually move farther and farther north. Thus whenever peak flows erode through the sand dunes to reach the ocean, the mouth will tend to return to the northern location eventually.

Levees and flood gates along the estuary block tidal waters from entering former sloughs. Former tidal flats are now primarily hay fields and pastures. This means there are less tidal eel grass beds to provide rearing habitat for juvenile anadromous salmonids. Because the levees stop tides from flowing into many areas, they reduce the volume of water moving in and out of the estuary. Prior to construction of the levees, a larger volume of tidal flow created stronger scouring action. Tidal scouring presumably maintained the deep pools that existed in the estuary. Now the scouring effect of tidal action is reduced and is evidently insufficient to recreate deep pools and channels in the estuary.

Artificial recreation of deep channels is possible. However unless the volume of the estuary is increases, the scouring action will not increase, and it may be insufficient to maintain the deep channels. They might fill with gravel again over time. A hyrologic model of the estuary might clarify this uncertainty. The model could estimate the amount of scouring action created by enlarging the estuary by different amounts. The model might show that partial restoration of the estuary will create enough tidal scouring to create deep pools.

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