My geography teacher used to say: “On small islands you won’t find rivers. There is a mnemotechnic verse to remember that: ‘On small islands you won’t find rivers’”
Don’t know if it is even true. Stuck with me to this day, if I ever meet him again, I will ask about whether small islands can have lakes or not.
So far the smallest island I've found with something people describe as a "river" is Suðuroy, in the Faroe Islands, at 163.7 km2 (63.2 sq mi).
"the great cirques of upland central Suðuroy where the hilltops reach an altitude of 574 m, but overlook and to a degree shelter the summer grazings of Hovsdalur (altitude about 200 m). The whole is drained by the Hovsá, a salmon and trout river. Freshwater is ample here as everywhere in the Faroe Islands, with rivers, count-less streams and an average annual precipitation of 1334 mm (Søgaard, 1996)." - https://link.springer.com/content/pdf/10.1007/s10745-005-474...
Is Suðuroy a "small island"? Perhaps! I found a larger island which Wikipedia describes as small.
"Rinca, also known as Rincah, Rindja, Rintja and Pintja, is a small island near Komodo and Flores island, East Nusa Tenggara, Indonesia, within the West Manggarai Regency", at 198 km2 (76 sq mi), says https://en.wikipedia.org/wiki/Rinca .
This is stretching it but this island (Poro Island - 96.6 km2) seems to have a "stream" and a waterfall into what looks like a pond.
I wouldn't really call it a river by any means though.
https://en.wikipedia.org/wiki/Poro_Island
Hmm. It appears to be artificial. The Wikipedia entry says "an artificial open channel" and 'Visit To Places Of Historic Interest In The Aberdeen Area Of Hong Kong Island' from 1967 (at https://www.jstor.org/stable/pdf/23890119.pdf) says:
"It was up this valley that Sir George Staunton, the eminent sinologue and Third Commissioner in the Amherst Embassy to Peking in 1816，strolled from the Aberdeen anchorage the following year to visit the village 一 in so doing to give his name to Staunton Creek now, 150 years later, being reclaimed from the sea."
I would not call that a river.
On the other hand, while researching the above I found that Lantau Island, Hong Kong at 147.16 km2 (56.82 sq mi) has the Tung Chung River.
I interpreted the joke being that a mnemotechnic is like a river, poetry flowing, so you won't find one for knowledge that is already concisely formulated, like a small island.
Don’t know if it is even true. Stuck with me to this day, if I ever meet him again, I will ask about whether small islands can have lakes or not.