Why is this verb in -masu stem form?


太陽が強く照らすから川の水がもうすぐ **無くなり** そうだ。

Can anyone explain for me why the verb in bold is in -masu stem form and not
fully conjugated?

(it's a sentence from a graded reader)

conjugation of the verb:
[https://www.tanoshiijapanese.com/dictionary/entry_details.cfm?entry_id=51120&j=%E7%84%A1%E3%81%8F%E3%81%AA%E3%82%8B](https://www.tanoshiijapanese.com/dictionary/entry_details.cfm?entry_id=51120&j=%E7%84%A1%E3%81%8F%E3%81%AA%E3%82%8B)

**Answer: Found a good explanation of this concept:**
<http://www.guidetojapanese.org/learn/complete/appearance>

