Website fingerprinting attacks enable a local adversary to determine which website a Tor user visits. In recent years, several researchers have proposed defenses to counter these attacks. However, these defenses have shortcomings: many do not provide formal guarantees of security, incur high latency and bandwidth overheads, and require a frequently-updated database of website traffic patterns. In this work, we introduce a new countermeasure, DynaFlow, based on dynamically-adjusting flows to protect against website fingerprinting. DynaFlow provides a similar level of security as current state-of-the-art while being over 40% more efficient. At the same time, DynaFlow does not require a pre-established database and extends protection to dynamically-generated websites.