San Francisco Bay turns brown after nonstop storms

Satellite images taken by NASA in April 2016 (right) and February 2017 show the impact of rivers dumping fresh water and sediment into San Francisco Bay. Satellite images taken by NASA in April 2016 (right) and February 2017 show the impact of rivers dumping fresh water and sediment into San Francisco Bay. Photo: NASA Photo: NASA Image 1 of / 5 Caption Close San Francisco Bay turns brown after nonstop storms 1 / 5 Back to Gallery

Have you noticed?

The dark emerald green waters of San Francisco Bay have turned a murky brown.

The Bay resembles the muddy Mississippi, and the paddle wheeler "San Francisco Belle" docked on the Embarcadero is looking more and more at home.

The tawny color is the result of fine sediment washed by back-to-back storms from the slopes of the Sierra Nevada and the soils of the Central Valley and into rivers.

These rivers are swollen and roaring from the heavy rains and their high flows whisk along the fine particles that are heavier than water.

They carry the fine silt into the Bay where its dumped along with surges of fresh water and is further stirred up by wind, tides and wave action.

A pair of before-and-after photos of the Bay, taken by a NASA satellite in April 2016 and February 2017 (above), show that sediment is floating around the waters of the northern half of the Bay and is fanning out under the Golden Gate Bridge into the Pacific Ocean.

"These photos are great because you're seeing this once-a-decade kind of event when we get very large flows on the two main rivers of the Central Valley — the Sacramento and the San Joaquin," said Jeffrey Mount, a senior fellow with the Public Policy Institute of California. "When these flows are high, they carry high amounts of very fine clay sized particles. That's what you're seeing."

Mount added: "It gives you a great look at how sediment mixes in the Bay. You can actually see the line between the high salinity and low salinity water that crosses above Angel Island. If you go out there in a boat, and measure salinity you will find that the brown water water is low salinity and the green high salinity."

While the dirty-looking color might not be appealing to the eye, the good news is that the powerful pulse of sediment and fresh water is largely a good thing for the overall health of the Bay.

The flushes of sediment help build up the mud flats and marshes at the Bay's edges. These are prime habitat for wildlife and provide barriers against rising sea levels.

"People who drive over bridges crossing the Bay at low tide, know the bottom of the Bay is exposed and we have these mud flats," said James Cloern, a senior scientist with the USGS. "If we took away the sediment, those mud flats would disappear. Those mud flats are important habitat for birds that migrate along the Pacific Flyway. They either stay in San Francisco Bay or use it as a resting area. There are about a million shorebirds that come to the Bay during spring migration. They feed on small shrimp and fish."

Beyond the mudflats, on the farthest edges of the Bay, is the marshland. Compaction leads this landscape to become more dense and sinks. Fresh sediment added on top of the buildup helps maintain the marshes.

"The Bay is surrounded by marshes and you need sediment in those marshes to build them up, especially as sea levels are rising," explained Mount, an emeritus professor at UC Davis in the Department of Earth and Planetary Sciences. "The marshland is great for birds and fish and really for everything because this is part of the physical habitat that was built by these upstream rivers. Historically the Bay had more sediment. In fact, it's sediment starved."

There are some downsides to the situation. Rivers are also collecting garbage and dumping it into the Bay and not all living creatures will survive the influx of fresh water and sediment.

"Not all the filter-feeding organisms will survive," said Dr. Rich Mooi, who works in the the Invertebrate Zoology Department at the California Academy of Sciences. "They don't eat dirt. The sediment clogs the filtration systems of the clams and things. It also changes the ability of bottom-dwelling plants to photosynthesize."

But the pros far outweigh the cons. "We really need this revitalization of organic material coming into the Bay," Dr. Mooi added.