According to the Oxford Dictionary definition 1, nature is a mass noun, otherwise known as uncountable. Under some circumstances, you can use a definite article with a mass noun: here is a good explanation of when not to do this:
If we want to refer to something general and the nouns that we are using are either plural or uncountable, we leave out articles. Definite and indefinite articles in English Grammar
No article is required for nature in your quote.
When you talk about a river, ocean or sea and you use either the name on its own or the name and designation, you normally use the in front of it. Strangely, this doesn't apply to lakes, ponds, brooks and streams.
The estimable Tᴚoɯɐuo may quote references saying that you can talk about the river Severn without a definite article, but I lived near the Severn for the first 20 years of my life, and I never heard anybody omit the definite artice.
I therefore think that you have a very strong case for saying that Qiantang River deserves a definite article.
You say "the Level 3 version of the article uses much more natural-sounding English". Level 3 probably uses news reports verbatim or with few changes. The Level 2 version will require rewriting rather than minor changes. The company that produces this web site is based in the Czech Republic and, while their efforts are commendable, it is clear that they are not native English speakers.