Tides and currents on the Bay
In San Francisco Bay the level of the two high tides and the two low tides for each day are generally not the same. In the summer the lower high and lower low tides are in the morning, so you often must avoid shallow areas. In the winter the higher high tide and higher low tide are in morning, and there is usually plenty of water.
At the OWRC it is easy to estimate the tide level: if the gangplank to the dock is level it is a very high tide, if it angles steeply downward it is very low tide. On weekends tide information is displayed on a white board next to the gangplank.
Predicted tide levels are are usually obtained from published tables, although they can be calculated on your PC if you have a program and the harmonic coefficients for your location. Water levels are given relative to the MLLW (Mean Lower Low Water) level; an average over several years of the lowest of each of the days two low tides. The extreme tide range for Sausalito is about -2.0 to 7.0 feet, but typically the tides range about 6 feet. Tide predictions can be obtained from the Tidelines tide prediction book on the desk of the OWRC. NOAA predictions are available on the Web .
Rowers are mainly concerned about low tides; they don’t like them because:
- their oars get tangled in eelgrass in many of the shallow areas, e.g. where boats are anchored, on the East side of the channel.
- they tear out the bottom of their shell by hitting rusty bolts sticking out of the bottom mud in shallow areas, e.g. near piers, the far houseboats and the Southern end of Strawberry.
- they hit partially submerged rocks that they never saw before, e.g. off the Belvedere Point.
- they can’t row in the far northern end of Richardson Bay.
You can still have a nice row at a low tide, you just have fewer options and you need to be more careful.
You can see current as a wake made by the water as it moves past piles and buoys. Anchored boats usually swing downstream in the current. If you stop your boat completely in the water any movement (if there is no wind) must be movement of the water itself, i.e. the current.
You might also note that boats and landmarks directly astern appear to be moving to one side or the other. This again is a sign of a current.
Our rower on the right is aiming the shell well in front of the anchor chain of a boat. The boat’s heading is the dashed line, and it appears to clear the anchored boat easily.
However, the rower has ignored the wake around the buoy, the direction the anchored boat has taken in response to the current, and the lateral movement of the shell. The shell is actually moving in the direction of the solid arrow. This is the shell’s “track”, how it is actually moving in relation to the land, and to things anchored to the land.
Note the the shell is still moving very straight and true through the water, this rower’s wake appears straight behind, but the water itself is moving to the left, and carrying the rower along with it.
Now it is clear what is going to happen. The shell is going to either hit the boat or become entangled in the anchor chain. By aiming the shell instead 20 to 30 degrees up-current of the anchored boat (less in mild currents), even pointing at the stern of next anchored boat, the actual track of the shell will pass safely through the gap.
Tidal currents are stronger in the southern part of Richardson, and can be obvious amongst the anchored boats south of OWRC.