This video posted by MBARI (Monterey Bay Aquarium Research Institute) is quite amazing. This past August, a submarine ROV (remotely operated vehicle) was in the head of Mendocino submarine canyon (~400 m water depth), offshore northern California, when a sediment density flow (turbidity current) occurred. The ROV captured the event on video and made measurements of sediment concentration.
These observations indicate that this flow had a lower layer that was more concentrated (but still very dilute, <0.1% sediment by volume) and an upper, less concentrated layer. In total, the turbidity current is estimated to have been >100 meters tall. Approximately 1:25 into the video there’s not much to see because the ROV is caught in the more concentrated layer — experiencing a sediment ‘black out’. Check out the GRL paper by Sumner and Paull that accompanies this footage.
This is really exciting because basic observations of these important processes are exceedingly rare. To my knowledge, this is the best video footage of an event like this.