Were the bridges crafted solely by magic, or with the help of magic? Maybe this is niggling over details, but for a bridge that's crafted exclusively from magic, I imagine something thats more of a land bridge or stone arch. Almost like the wizards just transmutated the surrounding bedrock into the bridge. On the flipside, a bridge crafted with the help of magic could still be made with carved stone or whatnot, and the magic acts as supernatural rebar, allowing the bridge to span longer than normal distances.

While I'm perfectly willing to change the map as you suggested in your sketch, at that scale (as I understand it) you won't be able to discern much of any detail in the island or the nearby gate towns. At best, you'd see that there was an island with canals, and that it has bridges, and that there's some gate-towns.

By the way, while I didn't explicitly say this, when I was designing the canals, I envisioned the thinner canals to be roughly a standard 2-lane road, and the thicker canals to be a 4 lane road.