🔍 Key Concepts

• Inland lock and dam light signals used on U.S. rivers • Difference between lock readiness signals (for entering) and wall/approach markers • Meaning of green lights in various combinations (one, two, or three) at locks

💭 Think About

• Ask yourself: Does a stack of three green lights usually tell you about the status of the chamber, or does it identify a specific structure/location near the lock? • Think about what signals would logically indicate "come ahead" versus "stand by/hold up" when approaching a lock. • Consider how mariners distinguish between the land wall and the river wall when approaching from downstream—what kind of marking system would make that clear at night or in reduced visibility?

✅ Before You Answer

• Verify what three green lights in a vertical line signify in the Inland lock regulations or local U.S. Army Corps of Engineers lock signal diagrams. • Check which signals specifically indicate "lock ready for entry" (often involving red/green change) versus those that simply mark physical features. • Confirm how the upstream ends of the land wall and river wall are each marked and whether they use different light configurations.