To + infinitive

[Guest]

Dear teacher,
Please help me explain why these sentences can be replaced with others containing "to+infinitive"
1. "we had a river in which we could swim"
- "we had a river to swim in."
2. " He was the first man who discovered this island."
- "He was the first man to discover this island."
Thanks a lot.

[Mister Micawber]

.
I am not sure that there is any more explanation than that there are often various English structures which can be used to express the same thing, in this case, purpose:

We had a river in which we could swim. (formal finite clause)
We had a river we could swim in. (informal finite clause)
We had a river to swim in. (informal nonfinite clause)
We had a river in which to swim. (formal nonfinite clause)
We had a river for swimming. (adverbial phrase)
We had a swimming river. (adjectival phrase)
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber