Reserve lands are owned by Canada and have been set aside (by her royal majesty) for the sole use and benefit of a band. This includes exclusive use and occupation. Interest in the land may be held by a collective First Nation (as Band Land) or an individual band member (as defined by the Indian Act) who has been granted an allotment to use and occupy the land as they wish. They have lawful possession of the land parcel within the reserve and are often issued a Certificate of Possession" and may be referred to as CP Holders.
Although legal title to the lands (Band or CP Holder) remains with the Crown, the Band and CP holders have the authority to do what they wish with the land, within the proviso of the Indian Act and other Canadian Acts and regulations, including restricting or preventing access to their lands. CP holders can even prevent the Band, their representatives or other members from accessing their lands.
If you are concerned about crossing Reserve lands, make sure you know exactly where the boundaries are. Download and install the Canada Lands NRCAN KMZ goodle earth plug in that shows all federal and FN land boundaries.
https://www.nrcan.gc.ca/maps-tools-publications/maps/data-canada-lands-surveys/11092Scroll to the middle of the page and look for the "Canada Lands in Google Earth" link.
I think many people will be surprised that they have been fishing on reserve lands. A significant example is the west side of the Capilano River, south of the rail crossing, which is owned by Squamish (it's part of Capilano IR5) and they could, if they wished, prevent anyone from fishing there. Another is the north side of the Vedder River near the On-the-Way Store.
If you have to cross reserve lands, as previously suggested, contact the band and ask permission. If you are polite, acknowledge that it is their land and you will be respectful and will leave the trail and river in a better condition than you find it, you will likely get a positive response. Unfortunately with bands like Soowahli, they have had such a long history of their property being abused, you may not have much luck there.
Regarding the OP's original question regarding Squamish land, there is a historic sensitivity to crossing Kowtain IR17, Aikwucks IR15, Waiwakum IR14 and Checkamus 1. I believe the Nation put up signs a few years ago (~5-6 ?) along a trail that provided public to access the Squamish River located west of Kowtain.
I don't know if the Canadian Navigable Waters Act provides protection to access water adjacent to a reserve as long as you can hike in from none reserve or fee simple lands. The act allows the public to travel freely
below the high water mark along all waterways that are navigable by canoe. Being a Canadian statute, I presume it would, but do not know for certain.