Passing Under a River Bridge: 10 Critical Checks for Boaters

Navigating under a river bridge requires situational awareness. Ten things every boater must verify:

1. Vertical Clearance — The #1 Killer

• Check charted clearance (MHW / MHHW) + tide stage + antenna/mast height.
• Rule: If clearance < your air draft + 1 m safety margin → do not pass.
• Bridge boards (gauge boards) show real-time clearance — read them.

2. Horizontal Clearance

• Channel span vs. your beam. Fenders, rub rails, overhangs count.
• Current can push you off-center — approach upstream, crab angle.

3. Structural Integrity

• Look for: Spalling concrete, exposed rebar, missing bolts, leaning piers, debris accumulation.
• Post-flood / post-impact: Assume reduced clearance until surveyed.

4. Overhead Utilities

• Power lines, fiber, pipelines often attached to bridge underside.
• Clearance may be < bridge structure. Marked on charts? Often not.

5. Current & Hydraulics

• Bridge piers = flow constriction → accelerated current, eddies, standing waves downstream.
• Scour holes at piers — depth sounder may show false bottom.

6. Weather & Visibility

• Fog, rain, glare (sun on water) → lose sight of bridge boards, channel markers.
• Wind: High bridges funnel wind; sailboats = heel + leeway.

7. Speed & Wake

• Slow speed / no-wake zones typical near bridges.
• Wake damages banks, docks, other boats; reduces your control.

8. Bridge Clearance Signs & Signals

• US: Green center span light = open channel; Red = closed/obstruction.
• Sound signals: 1 prolonged + 1 short = request opening (movable bridges).
• VHF Channel 13 (bridge-to-bridge) / 16 (hailing).

9. Emergency Egress

• Know where you can pull over / anchor if bridge becomes impassable (mechanical, traffic, weather).
• Upstream side usually safer (can drift back).

10. Local Knowledge

• Coast Pilot / Cruising Guide / ActiveCaptain / local marina.
• Idiosyncrasies: Tidal windows, bridge tender schedules, construction, events (regattas, fireworks).

Golden Rule

If in doubt, stop, assess, hail the bridge tender or coast guard. No schedule is worth a mast.