'I had new knees and my wife got new hips. One of the things that is really bad about the recuperative process is that we couldn't really handle a hike," says volunteer trail guide Henry Shenkman. "I talked to Fred Kramer, chairman of field guides at Mission Trails Regional Park, and told him I'd like to [organize] this kind of hike, called a nature stroll, on flat ground. It's designed for people who can't move very well and for people who are interested in going into [the subject of] nature more in depth." Shenkman's "Guided Nature Stroll at Old Mission Dam" stretches from the parking lot to the dam, or "about 75 to 80 yards in an hour." The dam was built to supply water via a flume, or aqueduct, from the San Diego River to the Mission of San Diego de Alcalá, located five miles from the dam. This was the first of the California missions, founded in 1769 by Father Junípero Serra. According to the National Park Service, "The precise dates of construction of Old Mission Dam, as well as the aqueduct and flume, cannot be ascertained...the dam was probably started in 1803, following a two-year drought. By 1817 it had certainly assumed its final form."

"The dam was built with primarily Kumeyaay Indian labor," says Shenkman. "It had a tile-lined flume, and there are still parts of it today that are hidden from the public so that they won't go out and destroy it." The park's visitor's center houses a replica of the flume. "On the south side of the dam you can see the layer of old tile and mixtures of different kinds of cement. They maintained the level of the dam by putting in wooden boards. On the other side of the dam you can see a small sort of slot for under the water wheel where they used to have a grist mill.

"A lot of people from the East who moved out here and retired miss the trees and miss the verdant forest," Shenkman says. "But then they come out here on the walk and see how diverse it is, and they become very interested...San Diego is the most bio-diverse place in the continental United States. We have over 73 different habitat zones in the county -- in Alaska there are 3." On this nature stroll alone there are five different habitats: hard chaparral, soft chaparral (coastal sage), grasslands, riparian, and aquatic.

During the month of December Shenkman points out galls to his tourists. "Galls are actually zits, like pimples, on leaves and trees where insects have laid their eggs. If it's warm enough you get to see bees, butterflies, maybe a wasp or two, and then by the water, later in the year when it's warmer, you see water striders and midges [tiny flies]."

Shenkman digs into the soil to unearth a few insects and keeps an eye out for lizards and birds. "You'll hear the wren tits; they sound like a ping-pong ball. We might see one or two raptors: red-tailed hawks, and there may be a golden eagle in the park.

"A lot of what we do is look and see where the sun is on this day of the year, how much the wind is blowing, how damp the soil is. Now we're into the beginning of the wet season, and a lot of things are just sitting there and waiting for the storms."

According to Shenkman, the four seasons in San Diego are the Santa Ana, the rainy season (in the winter), the stratus season (for the stratus clouds present during "June gloom"), and the monsoon season.

"We don't think about it much, but in [San Diego's] east county and Arizona and Nevada, when we get hot weather in the summer, there are occasionally big storms down in Baja that send a monsoon up into the area...some of the biggest rainfalls in the county occurred in August."

Shenkman continues, "Southern California is a Mediterranean zone, not a desert. In a desert you get less than six inches of rain, and we average eight to ten. A desert implies that there's nothing growing. There are only five places in the world that have this [Mediterranean] climate: the Mediterranean Sea, Patagonia in Chile and Argentina, Southern California and Baja California, Australia, and South Africa. They all have cold water-currents off the coast, [fresh] water, mountains, and are between 16 and 36 degrees latitude."

Shenkman begins his tours with what he calls the Goldilocks Principle of Ecology. "This is a theory of ecology I designed." Based on the well-known ending to the fable about a little girl and three bears, Shenkman says that for a "niche, or place in a habitat, everything has to be just right. " -- Barbarella

Guided Nature Stroll at Old Mission Dam Tuesday, January 3, 10 a.m. to 11 a.m. Old Mission Dam parking lot (north on Father Junípero Serra Trail at Mission Gorge Road) Mission Trails Regional Park Cost: Free Info: 619-668-3281 or www.mtrp.org