Why can you ride a flood but not an ebb?

When traveling from San Francisco toward Stockton or Sacramento, you can often ride the flood current for most of the trip. On the return, however, you cannot ride the ebb the whole way. Why?

The key is understanding that the tide is not a mass of ocean water flowing upstream. It is a wave moving inland. The water itself typically moves at 2–3 knots. But the crest or trough of the tide — the signal of high tide or low tide — travels upstream much faster, usually between 7 and 13 knots.

High tide occurs first at San Francisco. A few hours later it occurs at Crockett. Then Antioch. Then farther inland. The crest of the tide is marching upriver at roughly 10 knots. If your boat makes 6–8 knots through the water, you are moving at nearly the same speed as the tidal wave. You can remain inside the strongest part of the flood for many hours. You are essentially pacing the moving crest.

The ebb behaves differently in practice. Low tide also begins at San Francisco and moves inland at roughly the same 9–10 knot wave speed. But when you depart from the Delta on a strong ebb and head downstream, you are moving against the direction the tidal trough is propagating. You quickly outrun the strongest ebb, fall into weakening current, then slack, and eventually into flood.

The water flows south during an ebb. But the ebb wave itself moves north. That distinction explains everything.

Practical takeaway: When heading upriver, time your departure to move with the advancing flood wave. When heading downriver, begin near the start of the ebb at your location and plan to stop before the flood catches you. Do not try to “chase” the strongest ebb — it is moving the opposite direction.

Examples of the Speed of the Tide Wave

The tables below track the movement of high tide and low tide inland along the San Joaquin River system. Tide times come from tide stations. Distances were measured using Navionics Auto Route. The time difference between stations shows how long the tidal crest (or trough) took to travel that leg. Dividing distance by time gives the wave speed.

Notice that the wave speeds are far greater than the current speeds. The average flood-wave speed below is about 10 knots. The average ebb-wave speed is about 9 knots. The actual water current is typically only 2–3 knots.

Think of it like a stadium wave. The wave moves around the stadium quickly, but each fan only stands up and sits down in place. The tidal wave moves inland at 9–10 knots. The water mostly moves back and forth locally at a few knots.

When you head upriver on a flood, you travel with the moving crest. When you head downriver on an ebb, you are moving against the advancing trough. That is why you can ride a flood for many hours — but you cannot ride an ebb the whole way.