Learn Dutch: Waar is de rivier het smalst? - Where is the river the narrowest?

When a superlative (like 'the narrowest') describes a noun but does not come right before that noun, it always takes het. This happens regardless of whether the noun is a de or het word! So, we say de rivier is het smalst.

Would I ever say de smalste instead?

Yes, if the adjective comes directly in front of the noun! You would say de smalste rivier (the narrowest river) because rivier is a de word. But because the adjective is separated from the noun here, it goes back to the standalone form het smalst.

How do you form the superlative for smal?

You just take the base adjective smal (narrow) and add -st to the end. This is the regular way to make superlatives in Dutch, very similar to adding '-est' in English.

Why is het smalst at the very end of the sentence?

This is standard question word order. It starts with the question word Waar (where), followed directly by the verb is. Then comes the subject de rivier, and the descriptive phrase het smalst gets pushed to the end of the sentence.