Grinding Mortars and Water Holes
Monday, January 8, 2001
Copyright © 2001 Gary Valle'. All Rights Reserved.

It's raining today. That's good news, since there's been no significant rain in the Los Angeles area since October 29, 2000.

The flora of Southern California has adapted well to the unpredictable and drought plagued climate. The plant discussed in the January 1 Notes, California Prickly Phlox, is a good example. It subsists in rocky southwest facing crevices that may reach ground temperatures of 120 degrees or more in Summer. It is not surprising that it blooms in the Winter and has the needle-like foliage of many drought resistant plants.

Native animals have also evolved to cope with the capricious Southern California climate. Over time we will also have to meet the demands of our challenging weather. What about earlier cultures?

A few weeks ago, while running in the Simi Hills, I was exploring an area near a small, dry stream course and found a well shaded pothole that contained water. The pothole had been eroded in sandstone at a point where the stream threaded its way between some rocks.

That's not so remarkable, except that it had been over two months since there had been significant rain and the water in this five foot long pot-hole was up to 18 inches deep! (A couple of days ago it was still over a foot deep.)

Now, I always check the pothole when I'm in the area. On one of these trips two grinding mortars were found just a few steps from the pothole. These were presumably used for food preparation by a Chumash group. One of these is pictured above. It's easy to imagine that there would be periods when this stream was perennial, and periods when obtaining water was difficult, and water trapped in the "Mortars Pothole" was a valued resource.

The pothole is found in sandstone of the Chatsworth Formation. This is the same geologic formation described in Ponds & Pictographs. The soil that results from the erosion of the sandstone is quite porous and would appear to form a good aquifer. Sandstone can also absorb water and some combination of underlying structure, jointing, and a mini-aquifer seem to be a reasonable explanation for the longevity of the Mortars Pothole's water. In addition, in the Winter the pothole never sees the sun and is well protected from the wind. With the lower air temperature and lack of wind the evaporation rate is low. There also may be other factors, such as materials in or on the surface of the water, and the shape of the cavity.

Additional Notes

Summer 2001

As Summer 2001 progressed the Mortars Pothole became an important source of water for honey bees. During the day there was a constant stream of bees mining the water. According to bee expert Diana Sammataro, honey bees can ingest 0.36 to 0.5 milliliters of liquid per load. Based on the stream of bees going to and from the water hole, a bee would leave every 10-15 seconds. Over a 10 hour period this would amount to about 0.9 to 1.8 liters of water. ( A liter is equal to about a quart.)

Still damp and being worked by bees, the last driblet of water was gone from the Mortars Pothole on August 11, 2001.

After a long dry summer. there is water to be found, even in the arid foothills near Los Angeles. Some of these water sources, such as in upper Las Virgines Canyon are where you might look for water. But other sources spring from areas that you think could not possibly have water. The Chumash would have known them all, and in a way we can only barely appreciate.

Fall 2001

Following the passage of a couple of weak weather systems in early November and a stronger system on November 12, 2001, the Mortars Pothole was recharged and full of water.

Water Year 2001-2002

During the water year of 2001-2002, downtown Los Angeles experienced it's driest water year since record-keeping began in 1877. Only 4.42 inches of rain was recorded from July 1, 2001 to June 30, 2002. This was 29% of the 30 year seasonal norm. Even so, the Mortar Pothole had water through July 16! (This record has since been surpassed by the water year of 2006-2007.)

Fall 2002

Following 10 months of very dry weather in calendar year 2002, with barely more than an inch of rain, a wetter than expected storm that lasted from late in the day on November 7 through the morning of November 9, 2002 produced 2 to 3 inches of rain in the Simi Hills. Mortar pothole is full and appears to be fully recharged. The storm was stronger to the north with rainfall amounts in excess of 15 inches in some locations. There is a moderate El Nino that is forecast to last through February.

Water Year 2002-2003

Los Angeles (Civic Center) recorded about 16.5 inches of rain in water year 2002-2003. This was about 1.5 inches above the long term norm. With the increase in precipitation Mortar Pothole eked out an existance to September 1, 2003!

Fall 2003

There has been only about 0.2 inches of rain, and as of November 25, 2003 the Mortar Pothole remains dry. Finally! A storm Christmas Eve & Day recharged the mortar pothole system. Discovered large pond at the base of another slab and will investigate.

Water Year 2003-2004

Unofficially, downtown Los Angeles (USC) received 9.25 inches of rain, which is about 61% of normal. Much of Southern California recorded below normal rainfall, generally ranging from about 50-85%. Mount Wilson recorded 14.4 inches for the water year, which is only about 36% of normal. Mortar Pothole water survived until July 16, 2004, about a month and a half less than last year, and the same date as in 2002

Fall 2004

On October 17th an upper level low brought the first significant rain to the Simi Hills since early March. Unofficial rainfall totals ranged from 0.87" at the Cheseboro RAWS to about 1.34" in West Hills. This was enough to recharge Mortar Pothole, and revitalize the plants and animals of the area.

Water Year 2004-2005 & Fall 2005

The Los Angeles area was inundated with one of the wettest rain seasons on record. Downtown Los Angeles (USC) received 37.25 inches of rain, which is about 246% of normal, and the second wettest on record. As of August 18, 2005, there's still water in Mortar Pothole, and it persisted until September10, 2005. September thunderstorms in Southern California did not produce measurable rain in this area, but a storm system on October 17-18 did, fully recharging the Pothole.

Water Year 2005-2006 & Fall 2006

Rainfall in the Los Angeles area was a little below average for the water year, but the Mortar Pothole managed to hang on to September 25, 2006, and probably a day or two beyond. The Cheeseboro RAWS recorded 0.03" of rain on October 2, 0.12" on October 14, and 0.11" on November 27, but it wasn't until a storm December 9-10 when the area received about 0.5" of rain that the Pothole was recharged. (Cheeseboro RAWS rainfall data for this date appears erroneous.)

Water Year 2006-2007 & Fall 2007

From July 1, 2006 to June 30, 2007, Los Angeles recorded only 3.21 inches of rain, breaking the record set in 2001-2002 and making it the driest water year in Los Angeles since record-keeping began in 1877. Perhaps as a result of relatively cool summer temperatures, water in the Mortar Pothole persisted into at least the first week of August!

Surprise, surprise -- a record setting storm September 21-22 fully recharged the Pothole.