Not much security attention is given to the water inlets, but without these the dams are useless.  A necessary structure for the sluice gates along the Niagara River are large air vents; the black objects in the red circles.  These prevent vacuums from forming when the gates are closed, and also provide maintenance hatches.

A simple steel grate and lock prevent access to the diversion tunnel.  The Canadian side also has a chain link fence.  While many household products can be turned into explosives, and many types of explosives are available for legal purchase, terrorists have easy access to the good stuff like composition-A found in mortars and land mines.

A porous border and minimal security precautions now has fifty-pound explosive backpacks traveling through the diversion tunnels.  Their design is public knowledge, and they can be compromised.  A collapse of these tunnels would require years to repair.  The New Tunnel Project is also exposed.  However, the canal on the Canadian side offers some relief and could be repaired in months.

A car chase makes for a simple distraction to eliminate a random patrol by a police officer.  We've all seen these California chases on the news.  When the car is finally stopped, over thirty police cars are on scene.  That leaves a temporary security vacuum for the primary targets that only consume a couple of minutes, max.

Terrorists do not have to hit NYC directly.  Niagara Falls feeds power throughout the northeast.  Only four are required plus a single decoy to accomplish this mission.  These diversion tunnels represent resource-terrorism targets.