ADCPs rely on suspended particles, or scatterers, in the water column to reflect sound pulses back to the instrument. Suspended sediment, along with plankton and small bubbles, is an important contributor to the scatterers used by ADCPs, but too much can cause problems. If your ADCP is deployed in an area with a lot of mobile or suspended sediment, this can cover your transducers and affect the signal strength of the acoustic pulses.
Blockage from sediment can be broken down into two classes: mud and sand. Mud will attenuate the acoustic signal and therefore the performance will be dependent on how much the transducer is covered. Sand on the other hand is much worse. Sand tends to reflect the acoustic signal and a layer of only a centimeter is enough to have very poor results.