When I moved to Hawaii in 1983, I took an oceanography class at UH-Manoa from the chairman of the Oceanography Department, E.D. Stroup.

His fine lectures answered my questions about the ocean so precisely, I even remember where I was sitting when I learned certain facts.

I was front and center the day Stroup explained why Hawaii's sand beaches often change dramatically from season to season. Sand beaches are sediment in motion, changing with the size and direction of waves. Big waves move sand seaward, making beaches narrow. Small waves move sand shoreward, making beaches wide. This phenomenon is called longshore transport.

I'm reminded of that lecture often lately as I read and listen to laments about Kailua Beach. The formerly wide, white beach, the best in Hawaii in many opinions, is going away. The big-wave-little-wave thing isn't working.

There are several possible reasons for this. Some experts think a sea level rise due to global warming might be causing this unusual and, perhaps, permanent erosion. But no one knows for sure that the rate of erosion on Kailua Beach is increasing. This could be one of those natural spikes that happen occasionally, and the beach will eventually fix itself.

Or not. We have man-made structures along the beach that are themselves slowly changing. One is Kaelepulu Stream, a canal that once emptied into the bay but is now sand-choked and stagnant.

Whether the stream ever flowed much naturally is hard to say since it has such a long history of human manipulation.

Enchanted Lake was once a 190-acre body of water called Kaelepulu Pond, a thriving fishpond supporting mullet and other aquatic species. About 90 acres of marshland surrounded this clear pond.

Ancient Hawaiians diverted water in this wetland area for taro farming. Later, sugar planters dammed Kaelepulu Stream, filled the pond and then pumped 2 million gallons of water from it daily to irrigate fields. After the sugar industry left, developers in the 1960s drained water from much of the marsh, filled it with soil and built the community we now call Enchanted Lake.

Over the years, silt from yards, pollution from streets and sand from Kailua Beach collected in the pond and stream. Now these once deep, clear bodies of water are shallow and murky.

Since the stream has too much sand and the beach not enough, it seems logical to dredge the stream, dump the sand back into the ocean and let the waves sort it out. But since the sand in the stream possibly contains pollutants, when workers dredge its mouth, they pile the sand in the beach park. Then wind and rain gradually deposit it back in the stream.

Nature gets away with flushing polluted water and sand into Kailua Bay during storms like we had last week, and we can see the gloomy results: brown water, bobbing trash and warning signs about contaminated water.

Besides the clogged stream, some Lanikai houses and a boat ramp also interfere with the shoreline's natural currents.

State officials are working on solutions to restore the beach, but there are no easy answers.

When Stroup explained beaches in motion, he made one point perfectly clearly: Mess with longshore transport and you'll pay a price.

In this case the price is dear.