Depletion of fresh water used to level the locks. Big fresh water lake that citizens depend on for drinking water and agriculture, which hasn’t been filling up lately due to el nino. My first thought was, well then pump in sea water at the locks to level them. But that makes sense. Anyway, it’s a long discussion, explores the prospect of alternatives, gets into the history of it all.
Unfortunately, it takes the whole “climate change” thing as a given. Personally, I think we should be so lucky. If it were true, it would be wonderful. But I don’t think it is.
As you note, the Panama Canal is powered by rainfall which goes into lake Gatun. It was on the edge of the traffic it could handle when I was there 1985 - 1989. The USA had built another big dam further up the Chagres river to provide more water by then. Creating Lake Alajuela in 1935 almost doubled the water reserve available from the rivers. From An Ecosystem Report on the Panama Canal: Monitoring the Status of the Forest Communities and the Watershed :
... Panama Canal Commission maintained detailed stream flow records for many years, and PMCC assembled data on the annual water budget of the Canal. Total rainfall over the 3300 km 2 watershed averages 9 × 10 9 m 3 of water per year, of which an estimated 4.6 × 10 9 m 3 would be lost to evapotranspiration (Leigh, 1999), leaving 4.4 × 10 9 m 3 to flow into the Canal. With current ship traffic, 37 per day, more than half of this water, 2.6 × 10 9 m 3 , is used to fill the Canal’s locks – 191 000 m 3 per ship. An additional 1.2 × 10 9 m 3 of water is used to generate electricity for Canal operations, and 0.27 × 10 9 m 3 is processed for drinking water to supply most of the Canal communities and parts of Panama City and Colon. The three large rivers feeding Lake Alajuela (Chagres, Boquerón, and Pequení, see Figure 7) carried a mean of 1.7 × 10 9 m 3 of water per year from 1970–1996. The three largest rivers feeding Lake Gatún (Gatún, Trinidad, and Cirí Grande) carried another 0.73 × 10 9 m 3 of water, so these six rivers contribute 2.4 × 10 9 m 3 , 54% of the Canal’s water. In 1982, a dry year accompanying a strong El Niño event, they carried just 1.8 × 10 9 m 3 , a 25% reduction. If the entire watershed suffered a 25% reduction, the 4.4 × 10 9 m 3 of water typically available would become just 3.3 × 10 9 m 3 , less than the 4.1 × 10 9 m 3 needed to fill locks, generate electricity, and produce drinking water. A good deal of this can be made up by drawing down Lake Alajuela and the Canal, but in extreme years this means the Canal is too shallow for the largest ships to transit. Clearly, the water budget for the Canal watershed is tight enough that changes in runoff or sedimentation caused by land use are a serious ...