Sediments Sink River’s Flow into Sea

Pesticides, organic carbon, and other nutrients may be reaching the seafloor near continents via an unexpected route. They’re carried along the ocean bottom by dense, sediment-rich plumes of fresh water dumped into the sea by rivers, says a team of scientists.

The submarine Monterey Canyon (dark blue) cuts through the continental shelf off California and extends nearly to the mouth of the Salinas River (arrow). 1999 MBARI

Seawater typically is about 2.5 percent more dense than fresh water, so a river’s flow usually spills across the top of the ocean as it travels away from land. Normally, the river waters mix with the ocean to form a brackish blend, and any sediments carried by the flow settle out in shallow water to form the river’s delta. However, scientists studying California’s Monterey Bay have recorded five instances in the past dozen years when mud-choked swirls of unusually warm river water swept along the bottom of the bay at depths of nearly 1 kilometer.

These underflows were detected by tethered vehicles remotely operated from ships and by seafloor instruments, says Kenneth S. Johnson, an ocean chemist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. Sensors measured the water’s temperature and salinity and the amount of light transmitted through 25 centimeters of water.

Clear water cuts light’s intensity by about 10 percent. Typical muddy conditions dim the light by up to 40 percent. During some of the underflows, visibility dropped nearly to zero, Johnson and his colleagues report in the November Geology.

Each of the five underflows measured by the scientists occurred during a period of flooding in the Salinas River, which flows into Monterey Bay. The temperature and salinity of the water at those times suggest that the turbid flows should have been less dense–discounting the sediments–than the water normally found on the bottom of the bay.

The low salinity of the muddy torrent indicates that the suspended sediment rode within a mass of fresh water and didn’t simply rain down from the ocean’s surface, Johnson says. The researchers add that the most probable reason for the fresh water’s bottom-hugging flow is its immense load of sediment.

Scientists have seen smaller versions of these dense underflows–which are called hyperpycnal currents–at the mouths of the Yellow River in China and the Sepik River in New Guinea. But in those places, these currents were detected only near the shoreline at a depth of about 10 m.

Sensors show the hyperpycnal flows in Monterey Bay reach much farther. Johnson says that teams of researchers studying the seafloor 200 km off Monterey Bay at a depth of about 4 km have detected increased amounts of suspended sediment at times when Johnson’s sensors detected muddy currents.

Johnson and his team estimate that during a flood in 1995, the Salinas River carried as much as 3 million tons of sediment down Monterey Canyon each day. That tonnage, spread over a patch of seafloor 200 km on a side, would deliver about half that area’s normal annual total of carbon nutrients. Hyperpycnal flows could also transport significant quantities of pesticides because agricultural chemicals often hitch a ride on particles that wash into rivers.

Hyperpycnal currents are probably a major mechanism for transporting sediments from the continents to the seafloor far offshore, says Richard W. Garvine, an oceanographer at the University of Delaware in Newark. The link between the episodic, muddy surges that Johnson’s team observed and the flooding in the Salinas River will help scientists determine when they’re most likely to see such underflows, he notes.

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