The "winter bourne" is actually the baby Kennet, that in dry summers hardly makes an appearance.

Mr. Bourne was a slave until he was twenty-three years old.

When summer came, Our pastime was, on bright half-holidays, 55 To sweep, along the plain of Windermere With rival oars; [B] and the selected bourne Was now an Island musical with birds That sang and ceased not; now a Sister Isle Beneath the oaks' umbrageous covert, sown 60 With lilies of the valley like a field; [C]