Due to the size of the river cruise boats these are going to be small (even smaller than the luxury boats), cost more, and cater towards couples / pairs that are mature adults. One thing to note with river vessels is that there are not a lot of port spaces so they will dock beside each other and depending on how many boats wide this park job is, you may have to cross over another ship before you get to port. Also, since these ships are smaller and have popular itineraries, they sell out very fast!
Tauck
Known for: being one of the oldest operating tour companies in the USA also known for showing the experienced traveller the world from a different perspective
Aqua Expeditions
Known for: traveling the Amazon and Asian with environmental preservation / local experiences
Viking Cruises
Known for: being award winning, catering towards more active adults
Uniworld Boutique River Cruises
Known for having the beds in the stateroom face the window (ie. lie in bed and watch the world go by)
Ama Waterways
Known for: being voted the Best River Cruise Line in 2018 and partnering with Disney
Scenic
Known for: being more family friendly than the other lines
Pandaw River Cruises
Known for: getting off the grid in Southeast Asia
Avalon Waterways
Known for: being more catered to Western tastes and their food that gets a local flair from the cities you are travelling
CroisiEurope
Known for: the largest river cruise in Europe, it is French so food is a highlight
American Cruise Lines
Known for: modern river boats that travel along well known rivers in the USA & single travellers
Emerald Waterways
Known for: premium service