Fall at Walt Disney World is when things historically have quieted down as far as crowd numbers and activities. During this time (and right before the traditionally busy Christmas season) a number of attractions, shows, and dining locations tend to undergo some refurbishments.
Reports circulated online stating that the Liberty Square Riverboat will be closed starting August 24th and is estimated to return to normal operations on August 31st.
It appears this temporary closure is a new normal for the Riverboat this time of year. A similar refurb took place in 2019 and an 11-month closure happened in 2018.