Western United States: Lake Tahoe
Lake Tahoe is located in California, which is on the western side of the US, so is therefore commonly associated with the western United States.
Eastern United States: Ohio River
The Ohio River is in, Ohio (and some other states). Ohio is in the Eastern United states, so therefore the Ohio River is commonly associated with the eastern United States.
Canada: Hudson Bay
The Hudson Bay is north north, really up there. It's found in Northeastern Canada, above it, where Hudson explored with a crew, and got stuck in there. Eventually they got out, but the crew rebelled and threw Hudson and his son overboard.
Western United States: The Colorado River
The Colorado River is in Colorado, (and some other states) which is on the left side of the US, or the western side. Therefore the Colorado River is commonly associated with the Western US
Canada: Arctic Ocean
The Arctic Ocean is on the northern side of Canada. It is one of the only countries that touches it. (Some other ones are Russia, Greenland, etc.) Therefore the Arctic Ocean is commonly associated with Canada.