Why is it die Themse?

Themse ends in -e, and most nouns ending in -e are feminine: in a German dictionary, 79% of them take die.

All nouns ending in -e
Sense 1
die

Meaning

[1] English river that flows through London

[1] englischer Fluss, der durch London fließt

Examples

[1] They drove the Thames upwards with a boat

[1] Sie fuhren mit einem Boot die Themse hinauf.

Declension

1 Singular Plural
Nominativ die Themse —
Genitiv der Themse —
Dativ der Themse —
Akkusativ die Themse —

Dictionary entries include material from Wiktionary. Wiktionary