University of California Water Resources Center

In a natural tidal channel, Tule Slough, in Petaluma Marsh bordering the Petaluma River north of San Pablo Bay, synoptic measurements of stage and velocity were made at seven locations along the 3 mile length of the channel. Stage at each location was tied to a common datum and thus slope of the water surface could be computed. Values of slope were predominantly of the order of .0001 or less but under some tides and in one reach attained a value of .0006.

As maximum or minimum tide height approached slope became zero but often hovered near zero for long periods of time. With few exceptions slope became zero hours or minutes before velocity reversed direction indicating that an adverse slope is necessary to reduce the water momentum to zero.

Contrary to previously published accounts, none of which had synoptic measurements at several stations, the graph of slope vs time is not sinusoidal but much more closely resembles a flat and long cycloid curve.

The longitudinal profile of the water surface is practically horizontal at maximum tide for the full three miles of channel length. Due to shallowing of the channel upstream at lowest tide the upper reach becomes essentially dry while in the downstream reach the profile is nearly flat. At mid tide the water surface slopes in opposite directions at the two ends of the channel length.

The data suggest that there are at least two and possibly more types of tidal channels with respect to the change of width, depth, velocity And discharge along the channel. Tule Slough has few tributaries, has a nearly constant width for most of its length, shallows but little except at its far upstream reach. Therefore the hydraulic geometry relations found in the one channel for which data are available, Wrecked Recorder Slough in Virginia, do not apply. It is here suggested that such a difference would result from the character of the sediment making up the marsh and thus the type of vegetation growing on it. This pickleweed marsh is underlain by very fine clay containing much peat. The clay is derived from San Pablo Bay which is a shallow, large reentrant fed by only two minor rivers and which recieves [sic] little or no water or sediment from the Sacramento River.