The Shihmen Reservoir, completed in early 1960s, has been an important hydro project in Northern Taiwan. Soil erosion and sediment have been a major concern for the longevity of the reservoir. After a series of typhoons in 2004, the intake valve of the hydro power plant was covered by 10 m of sediment. The power generation has been halted since then. The intake valve was originally designed to be operated in clean water. In order to evaluate the feasibility of re-opening the power plant intake valve, it was necessary to know the density state of the sediment (referred to locally as the bottom mud) and the lateral pressure exerted on the intake valve. The center of the intake valve was at approximately 70 m below water. A testing device that consisted of a time domain reflectometry (TDR) probe placed on top of the Marchetti dilatometer (DMT) was developed by the authors with an original intention to determine simultaneously, the solid concentration, stiffness and stress state of the bottom mud. The TDR/DMT probe was attached to a string of 90 m long drill rods. A skid mount drill rig bolted to a barge was used to control the drill rods. The weight of the drill rods was sufficient to push the TDR/DMT probe into the bottom mud. TDR and DMT readings were taken from 60 to 80 m below water. The electrical conductivity measurement from the TDR probe was used to determine the solid concentration. The lateral stress was inferred from the DMT po readings. The difference between po and p1 was used to determine the density state of the bottom mud. Ten DMT profiles were taken five of them had TDR readings. The paper describes field set up of the TDR/DMT probe, its test procedure and interpretation of the test results.