Geographical names have to be capitalized
http://cafsclasswritingcenter.weebly.com/capitalization.html (see section D)
https://www.dummies.com/article/academics-the-arts/language-language-arts/grammar-vocabulary/when-do-you-capitalize-geographic-terms-190456/
Specifically:
https://en.wikipedia.org/wiki/Serengeti
https://en.wikipedia.org/wiki/Central_Asia
https://en.wikipedia.org/wiki/Rubicon
https://en.wikipedia.org/wiki/Indus_River
Wikipedia is itself inconsistent on the rivers/Rivers. I suggest capitalizing them.