Do you make a bracha when seeing Niagara falls, and, if a bracha is made, which bracha?
Some believe that an "Oseh Ma'aseh V'reishis" should be made on the Niagara Falls because undoubtedly it is one of the most impressive natural wonders. However, after doing some research I have found that it may not be so simple because of three reasons.
1) Rav Chaim Kanievsky is quoted (V'zos HaBrachah pg. 155) saying that waterfalls do not receive a brachah since they are not in the list that the shulchan aruch gives (mountains, seas, very large rivers etc.). The sefer V'zos Habrachah comments that even so, if the waterfall is part of a somewhat large river then it may combine with the river to become a very impressive river and then deserving of a brachah. The discussion is continued in the sequel which I unfortunately do not have access to.
2) The Shulchan Aruch says that a river which has been diverted by people can no longer receive a brachah that praises Hashem for his work, because it is no longer only the work of Hashem. Niagara Falls has undergone changes for the purpose of hydro-power and tourist-attracting aesthetics. But, it is unclear how much change must occur for a river to lose a brachah and this might not count as change.
3) Rav Shlomo Zalman Auerbach said not to make a brachah on the Dead Sea because of the medrash that it was only formed after the destruction of S'dom and the brachah "oseh ma'aseh v'reishis" should only be said on what was part of B'reishis- Creation. It is known that the falls erodes upstream a couple feet every year. 800 years ago there may have been only one waterfall. If the erosion theory is correct, Niagara Falls at Creation was nowhere near where it is today.
So I say a brachah on Lake Ontario and have in mind to cover the Falls. This method might work. One who lives near Lake Ontario, so is desensitized, might not be able to make a brachah on Lake Ontario either and only be able to channel feelings of awe and inspirations through a brachah without Hashem's name.
I believe one makes the bracha "oseh ma'aseh bereishit" upon seeing wonders of nature. Here is a discussion of Niagra specifically
Since the question does not address which berachah/ot OP is speaking of I’ll address another relevant berachah: seeing a rainbow.