From the Dam Safety website:

Dam Safety 2021 will return to an in-person format in Nashville with an On-Demand component. During registration, attendees will be able to select if they are joining us in Nashville (with full access to all On-Demand content following the conference) or if they will be accessing the content On-Demand only. All conference sessions, including two general and 38 concurrent sessions, as well as 20+ lightning talks, will be recorded and available On-Demand to registered attendees starting October 1. Plus, 14 presentations will be available On-Demand only. All On-Demand content will be available for six months.