A subscription list is an OPML file in the lists folder of your River4 data folder.
Each list corresponds to a river. The feeds in the list are the feeds we read to find new items for the corresponding river.
Suppose you wanted a river of news about movies. You'd create an OPML file in the lists folder called movies.opml. River4 would read the feeds in that list periodically, and put the new items in the riverjs file in the rivers folder. The river file is called movies.js.
To edit the list you can use a text editor, to hand-edit the OPML, or you can use an outliner whose native format is OPML.
My outliner, Fargo is perfectly suited for editing OPML subscription lists.
First create a new outline, using the File/New command in Fargo.
Create a new headline by pressing Return. Type a short description. Choose Add Feed in the Outliner menu, and paste the URL of an RSS or Atom feed. Repeat this for all the feeds you want to add to your list.
If you want to include another OPML list in this list, press Return to add a new headline. Type a short description, then choose Add Include in the Outliner menu, and enter the URL an OPML file. When River4 processes the list, it will be as if all the feeds in that list were in this list. Here's a Fargo docs page on includes.
You can use the include feature in the previous step to create a placeholder list in your lists folder. Just have it include the list you're editing in Fargo. You can get the URL of the outline by choosing Get Public Link in Fargo's File menu. This is a bit complicated, but worth studying and understanding because it makes Fargo a simple way to tell River4 what feeds you want it to read.
It's easy to have a second or third river, just create another OPML file in lists folder. It's probably good to get started with a single list, and get a feel for how busy the feeds are, how many new items you get every time you look at the river.