1. It derives from the Iroquois word Onyahrah, or 'neck', a name which the Iroquois applied to the neck shaped land (or peninsula) that lay between the two lakes - Erie and Ontario.
2. Some scholars think Niagara is named after the 'Onghiahrah' tribe.
3. Most scholars contend Niagara comes from the Huron word Oniahgahrah, or 'Thunderer of Waters'.
Here is an excerpt from 'Niagara Township, Centennial History' that refers to these meanings:
"Some scholars think the word Niagara comes from the name of that Indian tribe, the Onghiahrahs. Others think it derives from the Iroquois word Onyahrah, or neck, which the Iroquois applied to the peninsula (or neck) between the two lakes.
In any case, the word Niagara was applied first to the world-famous falls, then to the equally famous river, and still later to Township Number One, now better-known as Niagara Township."
First, one may speculate that the word 'Oniahgarah', since it sounds like a cognate of the Sanskrit 'Nira-Gagarah' (नीर-गागरा) may have some connection with Sanskrit. The meaning too is fascinating. 'Nira' (नीर) means 'water and 'gagara' (गागर) means 'pitcher' or 'vessel', ' Nir-Gagarah, therefore would mean 'Gushing Water' and comes close to the Huron meaning 'Thunderer of Waters'.
Second, the second syllable of the word 'Oniagarah' is a cognate of the Sanskrit word 'jarah' meaning 'waterbody' or 'waterfall'. Uncannily cognates of the Sanskrit 'jarah' appear in the names of many rivers around the world such as 'Jordon' in the Middle East and the 'Jarra' in Australia. A cognate of jarah also appears in the name of river Nigeria. The second syllable in the name Nigeria carries the same meaning - waterbody or a fountain of waters waterfall. Hence, if there is a Sanskrit connection to the name Niagara, it may simply be derived from Sanskrit 'nirjhar' (निर्झर) which means 'Waterfall' !!
In India itself, one of the major tributaries of the ancient Saraswati River (on the banks of which ancient India developed) was the 'Ghagara'. The Ghagara today flows as an intermittent river and is known as the Ghaggar or Ghaggar-Hakkar.
There is yet another Himalayan River by the name of Ghagara that flows into the River Ganges.It originates in Tibet near Mansarovar, and crosses into India, via Nepal.
Could it be that the Asians who are believed to have crossed into America about 30000 years ago originally named the Niagara River as 'Nira-Ghagara'. Or maybe having found a new home the Native American gave the river a name which was the equivalent of New-Ghagara! Even New-Ghagara would translate as 'Nav-Ghagara' in Sanskrit!
And here is an observation about the legacy of the Native American names of the rivers of America that Edward Moor made in his book 'Oriental Fragments' in 1834, "In America what fine names might probably have been left of the vast lakes and streams and hills, which ennoble, beautify, and enrich those extended regions. How poor and uninstructive are the Hudson, the St. Lawrence, in comparison with Niagara - pure Sanskrit I suspect....".