Approximately 156 words in this 565-word poem function as gerundives. That is 28 % of the total words. The word "and" used to compound these gerundives comprise another 128 words (23 %). The result is a humorous, writhing and ringing, rattling and battling, guggling and struggling descent down the Cataract of Lodore. Put differently, this is an exercise to be used primarily for fun. 
     You might want to have the students analyze a few sentences from the first two stanzas (See the analysis key.) if you think there is something in them that the students might find to be a helpful review. But having the students formally analyze the majority of the poem would turn a good time into boring busy-work. My suggestion would be to make an overhead of the text of the poem and have the students simply read and discuss it in class. 
      You might want to follow the discussion by inviting students to write their own versions --  "After Southey's 'The Cataract of Lodore'." There is, indeed, an entire genre of "After ______." poetry. Poets would simply write poems in the style of, or in mockery of, other well-known poets. You can find a number of examples in the Fourth Volume of Burton Egbert Stevenson's Home Book of Verse which is available at Project Gutenberg. (The version of "The Cataract" used here is from the Third Volume.) Students will probably enjoy describing their favorite halfback slipping and sliding, driving and diving. Or they may prefer a ballerina tip-toeing and bow lowing. (If Southey could bend words out of their normal functions, your students should have that poetic right as well.) A large part of the humor of Southey's poem derives from the sheer number of gerundives, and if your students try to mimic that, they will also be getting some good vocabulary exercise. Don't forget to have students share their work with the class, and you can also submit them for publication here.