Lewis Carroll was rowing on the Isis when he first told the Alice story in 1862.
The Isis is the traditional Oxford name for the River Thames, especially the Thames as it passes through and south of Oxford. On 4 July 1862, Charles Lutwidge Dodgson—better known as Lewis Carroll—rowed upstream from Folly Bridge toward Godstow with the Reverend Robinson Duckworth and the three Liddell sisters: Lorina, Alice, and Edith.
During that outing, Dodgson told the children an improvised story about a girl named Alice. Alice Liddell later asked him to write it down, leading to the manuscript *Alice’s Adventures Under Ground*. Carroll expanded that manuscript before publishing *Alice’s Adventures in Wonderland* in 1865.
The most likely wrong answer is the Thames, because the Isis is not a separate river. It is an Oxford naming tradition derived from the river’s Latin name, *Tamesis*. Thus “the Isis” is the expected historical answer for the Oxford setting, while “the Thames” identifies the same waterway in its broader or official usage.