Resources

NOAA tutorial on tides... essays on tides and currents by your webmaster...

NOAA tide level predictions for Sausalito.

NOAA tidal current predictions for the station about 400 yards South of Yellow Bluff.

Water levels: Tides commonly refer to daily changes in water levels. The gravitional pull of the moon is the major force causing tides and thus the basic tide cycle, two highs and two lows, repeats every "lunar day", about every 25 hours. Thus the high and low tides are about one hour later each day. The sun also causes tides, but since the period of apparent rotation of the sun around the earth is 24 hours, the pull of sun and moon are sometimes along the same line, and sometimes at right angles to each other. The tidal range, the difference between high and low tide, is greatest when moon and sun pull in the same or opposite direction, at new and full moons. The range is also highest in June and December when the earth's rotation brings Sausalito closest to the plane of the moons rotation.

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.

NOAA has a comprehensive tutorial on tides. More idiosyncratic essays on topics specific to the Sausalito area can be found on your webmaster's website.

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 week ends 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 coeficients 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 tide range for Sausalito is about -2.0 to 7.0 feet. Tide predictions can be obtained from most newspapers and the Tidelines tide prediction book is on the desk of the OWRC. NOAA predictions are available on the Web (see sidebar on left).

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, e.g. to get into the Strawberry canal.

You can still have a nice row at a low tide, you just have fewer options and you need to be more careful.

Tidal currents: For the water level to go up and down in the Bay, water from the Pacific Ocean must come in and out through the Golden Gate; this is the tidal current at the Golden Gate. Similar currents are generated throughout the Bay. As the water level increases water flows in through the Gate, the flood; as the water level decreases water flows out of the Gate, the ebb. You might think that when the tide level was at a high or a low the current would stop, the time of slack. After all, that should be the time the current should be reversing in direction. However, there is a time lag, slack is actually about 90 minutes after a high or low. Tidal currents are predicted by NOAA in the same way water levels are. See the sidebar at the top of the page for links.

Rowers are especially interested in tidal currents if they are rowing outside of Richardson Bay, e.g. around Angel Island. However, even inside Richardson Bay it is important to be aware of currents, which can be more than 2 knts at maximum ebbs and floods. If you are rowing across a 2 knt current and want to pass in front of a boat heading into the current you need to aim 20 to 30 degrees ahead of the boat. If you aim where you want to go you will hit the boat.

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.

Our rower on the right is aiming the shell well in front of the anchor chain of a boat (dashed line). 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.

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.

Your webmaster was swept into the Green 3 buoy off Belvedere Point by a 3 knt current because he was looking at a speed boat that seemed to be intent on running over him and didn't notice the velocity of the current. (Some) men evidently aren't so good at multitasking.