To enter Canada by land, you only need your passport. If you change your mind and fly to Canada from anywhere, you will need to apply for an eTA for $7 from this official site:
The eTA is an authorisation to board the plane. No eTA, no flying to Canada.
To get back in to the USA, you will already have your ESTA for flying there in the first place, so with your passport you will be good to go.Edited: 15 May 2018, 18:49
Think of the American ESTA as an equivalent to our eTA - authorization to board a plane.
Almost all visitors to the US require a Visa. Citizens of the UK are eligible for a "Visa Waiver"...the I-94W.
The process has been streamlined, so it appears almost seamless at the Point of Entry.
No concern, as the US airports electronically "attach" the I-94W to the traveller's passport.
I hope that clears things.
Mufti has it correct
if you fly to the US on a passport that does not require a "visa" one needs to apply for an ESTA
if you fly first to Canada on a passport that does not require a "visa" one needs to apply for an eTA
for a land crossing from Canada to the US and you are from a "visa" free country you get an I-94 waiver at the land crossing.
when entering Canada from the US with a UK passport you only need your passport no additional documentation.
when returning to the US after a visit to Canada and you already have an ESTA because you flew to the US first you do not need a I-94 waiver to get back to the US; but there is a 90 day clause that applies.
Thanks, I’m familiar with what an ESTA is. I am aware of all that you are trying to “clear up.”
Maybe ask poster 2 what he means. Post 2 says the OP needs an I-94W. There is no evidence that the OP needs that. Based on what the OP says -
1. Visiting Michigan from the UK = ESTA. I am assuming a flight would be the way the OP will arrive in Michigan from the UK. 🤔
2. Crossing to Niagara Falls, Canada = passport, nothing else
No evidence that the OP is returning to the States, but if returning by land, the I-94W will already be electronically attached to the OP’s passport. If returning by air, the ESTA will already be in place.
Post 2 would only be accurate, IMO, if the OP came to Canada first (in which case there would be no need to cross into Canada to NF) and planned to drive to Michigan (requiring an I-94W at the border.)