Many viscosity measurements were made on samples of "fluid mud" from the Rappahannock River of the Chesapeake Bay system with an eight-speed rotational viscometer. Measurements were made directly in the core liner immediately on retrieval aboard ship; in the laboratory under various conditions of salinity, concentration, and temperature; and under conditions of slow sedimentation where the changing sediment density was simultaneously monitored with a nuclear density sensor.

Rheograms developed over a broad range of shear showed that the mud exhibits non-Newtonian behavior, usually pseudoplastic but occasionally dilatant, depending on settling time, concentration, and interstitial water salinity. At times both pseudoplastic and dilatant behavior were observed in the same sample at differing rates of shear. During increasing shear the apparent viscosity profile usually exhibited its highest values and most variable behavior. During decreasing rates of shear, the profile generally showed a hysteresis effect, indicating the mud to be thixotropic.

Differences in rheological behavior were noted between samples from the fresh, intermediate, and highest salinity reaches of the estuary. A steep decrease, followed by an abrupt increase in apparent viscosity at low shear rates, seemed to be common in intermediate-salinity reaches (2 to 8 parts per thousand), but similar behavior was rare in samples from freshwater and high-salinity reaches. This "viscosity notch" may help to explain the sudden burst of suspended sediment that has been observed as occurring off the bottom, just after slack water in intermediate-salinity reaches in the Rappahannock.

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