I found an old and now closed thread on this subject and I'd like to add some details.
I can't speak for river journeys but if you're planning to visit Burg Eltz and need to store luggage, here are current (2024) details.
To hike up to Burg Eltz (beautiful!), you start at Moselkern but there no official place to store luggage (except maybe at a friendly pub). One known option is to go to Cochem (16 minutes by train south of Moselkern). By the way, was traveling by train from Trier to Koblenz. There's a restaurant (Gaststätte) in the Bahnhof called "Gleis 9". The entrance is actully from th street side. They are open Thursday-Tuesday (closed Wednesday) from 10:00 am until 6:00/7:00 pm. Luggage Storage (Gepäck Aufbewahrung) costs 2.50€ per day.
Here's the catch. The RE81 from Trier runs once an hour and stops in Cochem at (for example) 9:43 --15 minutes before "Gleis 9" opens. The RE81 back up to Moselkern leaves at 10:44. So, if your plans are for an early start, you'd have to wait for it to open and thereby miss the train back, but then wait 45 minutes. It takes an hour to drop off your luggage.
I hope this helps you plan accordingly.