This piece uses rolling arpeggiated figures to depict the flow of the Severn river. It is distinct for its pastoral, flowing melodic contour and evocative, imagery-driven writing.
The challenge lies in coordinating the flowing left-hand figures with a rubato-heavy, expressive melody.
Comparable difficulty and character — good candidates for what to learn next.
'Twas on the Severn's sedgy margin is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 5.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 'Twas on the Severn's sedgy margin lasts about 4 minutes.
'Twas on the Severn's sedgy margin is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include 'Who Knows?' by Ernest Roland Ball, Canto del Bardo by José Ferrer, Blumenklage by Gustav Lange — see the full similar-pieces list on this page.