Moraga Watershed

Sanders Canyon: A Small Watershed in Moraga, California

Richard L. Gunther
May 1, 2006

ES 551 Computer Mapping Systems

General Map of Moraga

Contra Costa County GIS
Sanders Valley Watershed Highlighted in Yellow
Moraga is a small, bedroom community located about 10 miles east of Berkeley, California. The town's historical roots go back to 1835 when it was part of a Mexican land grant called Rancho Laguna de los Palos Colorados (Ranch of the Lake of the Redwoods)(Kimball, 1987). See Town of Moraga History. Today, the Town of Moraga has grown to a population 16,000 people.

Moraga sits within a watershed that reaches to Orinda in the west and Lafayette in the north. There are several small streams that generally flow south into San Leandro Reservoir. All of these small streams have steady flow during the rainy winter and spring months. Since this area has a Mediterranean climate with hot, dry summers, stream flow is greatly reduced in the summer and fall.

A small portion of the Moraga watershed is highlighted in yellow. This highlighted area locates the Sanders Valley watershed. Sanders Valley is occupied by homes and streets and is enclosed by a ridge of hills.

Specific Location of Sanders Valley Watershed

DOQ-UC Davis
Sanders Valley Watershed
Sanders Valley watershed has a distinctive delta shape. The southern boundary rests on a low ridge that is visible in the lower portion of the image. The ridge is oriented in a northwest-southeast direction for approximately 800 meters. The eastern boundary runs almost due north-south along the right-hand side of the image. The eastern boundard is approximately 1200 meters in length. The northwest boundary is oriented in northeast/southwest direction for approximately 1200 meters. The northwest boundary connects to the southern boundary in a creek that runs east/west through the valley. The three boundaries of the wateshed produce the triangular shape.

The perimeter of the watershed is approximately 3.5 km long. Slight variations in measuring the perimeter was caused by the location of homes, fences and streets.

The image on the left was produced using IDRISI function called Window. Using the Window function the view was taken from the Las Trampas Ridge DOQ 7.5-minute California quadrangle. The specific area was delineated using the appropriate columns and rows. A DOQ is an aerial photograph that has been corrected for distortions so that it now contains the spatial accuracies of a map.

Views of the Watershed

Southern Boundary

View of Southern Flank looking West
(Image by R. Gunther)
The southern boundary of the watershed is a low ridge. The low ridge is approximately 800 m long. The highest point on the ridge is located in the upper left corner of the image. From a high point of 256 m the ridge descends to 202 m. At that point the ridge quickly ends descending to the creek which has an elevation of 152 m.

The top of the hill is covered with a variety of grasses. There is a steep scarp that runs the length of the ridge which is covered with trees. The trees include live oak, bay and buckeye. Below the tree zone is a broad, grass covered slope that ends in the main creek that runs through the valley. Riparian vegetation seen along the lower right portion of the image marks the location of the creek.

Eastern Boundary

View of Eastern Flank looking South
(Image by R. Gunther)
The eastern boundary of watershed has a U-shape. The southeast corner of the boundary marks the upper part of the "U". This high point has an elevation of 256 meters and marks the intersection between the eastern and southern boundaries. The lowest part of the "U" is a saddle that has been modified by a road in a housing subdivision. The saddle's elevation varies from 216 to 218 m. The northeast corner is the other part of the "U". This point marks the highest point in the watershed with an elevation of 326 m.

The saddle which is now capped by a road can be seen in the image to the left. On the west facing slope of the saddle there are several clusters of willow trees that mark the headwaters of a creek. The unnamed creek flows westward from the saddle the length of the valley just south of the homes.

Northwest Boundary and Summit of Watershed

View of Northern Flank Looking Northeast
(Image by R. Gunther)
The northwest boundary is the highest portion of the perimeter of the watershed. The summit has a large, rounded top at 326 m above sea level. From the summit the ridge gradually descends in a southwest direction until the low point (152 m) is reached in the creek.

Several fingers of the main creek originate on the slope of the northwest boundary. The fingers are marked on the hillside by riparian vegetation. Enough winter rainfall is collected and stored in these v-shaped valleys to allow trees to grow. The rounded ridges and southfacing slopes can only support a variety of grasses.

Primary Creek in Sanders Valley Watershed

Primary Creek at 152 m of Elevation
(Image by R. Gunther)
Sanders Valley watershed is drained by a creek that flows in an east to west direction. The creek begins on the eastern flank of the watershed and traces a path immediately south of the homes on Sanders Drive. Flow is greatest during the wet winter months. Flow slows to a trickle in late summer and fall.

At the time (April) this image was captured the creek was a few inches deep and about 3 feet across. This flow is typical for the creek several days after a winter storm has brought precipitation to the watershed. The water is slightly cloudy because it is transporting a small amount of clay. Clay is the primary component of the adobe soil which dominates the watershed.

Summary View of Sanders Valley Watershed

View from Highest Point in Watershed Looking Southwest (Image by R. Gunther)
The image at the left nearly captures the entire Sanders Valley watershed. Rainfall that does not percolate into the ground would runoff downslope and be funneled into the creek. The creeks natural path through the valley has been modified by the suburban development. Eventually, the creek flows into San Leandro Reservoir which is located just behind the first ridge in the middle distance. The reservoir is part of the East Bay Municipal Water District. East Bay provides drinking water for Moraga and surrounding communities.

As this image highlights, the floor of the watershed has been converted into houses and streets. Conversion has altered the natural processes related to the watershed such as evaporation, percolation and runoff.

Images of the Watershed

Looking northeast into the Watershed
This three-dimensional image produces a nice functional view of the watershed. The three boundaries that make up the perimeter of the watershed are visible. The southern boundary is dark line visible in the front right hand corner of the image. The eastern boundary with its "U" shape is seen as a dark curving line in the rear of the image. The northwest boundary and near summit is visible in the left hand portion of the image. The higher summit to the rear of image is not part of the Sanders Valley watershed.

The image was produced by the IRISI program called ORTHO. A portion of the DEM from the 7.5-minute Las Trampas Ridge was used as the input image.

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DEM-UC Berkeley
DEM for Sanders Valley Watershed

The shape of the watershed is quite apparent in this Digital Elevation Model image. The sounthern boundary looks like a small, right hand thumb pointing westward. If the southern boundary looks like a right thumb, then the eastern boundary would be the palm of the right hand. The northwest boundary would be represented by the fingers. The total boundary forms a reverse "C" as would be formed by the right hand. The reverse "C" acts as a glove which catches the precipitation and funnels it into the valley.

The image was produced using IDRISI Window. Using the Window function the view was cropped from Las Trampas Ridge DEM 7.5-minute California quadrangle.

Mapping the Perimeter of the Watershed

On the afternoon of Thursday, April 6, 2006 the author undertook the collection of data which then could be used to precisely plot the perimeter of the Sanders Valley watershed. The data collection process involved hiking the perimeter of the watershed. During the hike a Garmin eTrex Vista was used to collect latitude, longitude and elevation readings. Latitude and longitude were recorded to the nearest 0.001 of a minute. Elevation data was collected to the nearest foot.

Sixty sets of data were collected between 3:00 pm and 5:30 pm. Each set (latitude, longitude & elevation) was collected every 50 to 60 linear meters. The total perimeter traversed was 3.5 km. A sample of data collected is located in Table 1.

The Garmin eTrex Vista displays latitude and longitude in degrees and decimal minutes (DM.m). The author utilized a spreadsheet to convert the latitude and longitude from degrees decimal minutes to decimal degrees (D.d). This conversion was necessary for the data to be properly interpreted by GIS program for the next map. The far right columns of Table 1 contain the converted latitudes and longitudes. An explanation of this conversion can be found at the following web site: Slow The Spread. The elevation data was converted from feet into meters. Converions were rounded to the nearest whole meter.

Latitude (DM.m)Longitude (DM.m)Elevation (m)Latitude (D.d)Longitude (D.d)
37°49.705 N122°7.397 E
200
37.82842° N-122.12328°
37°49.688 N122°7.381 E
206
37.82813° N-122.12302°
37°49.680 N122°7.363 E
212
37.82800° N-122.12272°
37°49.668 N122°7.341 E
218
37.82780° N-122.12235°
37°49.660 N122°7.334 E
216
37.82800° N-122.12223°
Table 1: Sample of Data Collected on April 6, 2006

Shaded Area Highlights Watershed

Sanders Valley Watershed
The image on the left shows the perimeter and area of the watershed. The perimeter was assembled using the 60 sets of latitude and longitude the author collected on April 6, 2006. The data appears as little dots on the topographical map. The shaded area represents the area of the watershed.

It was the goal of the author to generate this map using Idrisi GIS. Unforunately, the author did not solve the many logistical challenges that is associated with such an undertaking. So, the image contains the author's data but was not assembled by the author.

The map image on the left was quickly assembled by Mr. Dave Colby, a GIS specialist, who is employed by Modesto Irrigation District. MID Mr. Colby downloaded the 60 sets of data into his computer. He used the ESRI GIS program called ArcView to make the map. The author was able to observe the process and appreciated Mr. Colby's skill and knowlege. The author would like to thank Mr. Colby for assisting the author in this learning activity.

Annual Precipitation Within the Watershed

My father, Victor Gunther, has kept meticulous account of the annual precipitation in the Sanders Valley watershed for 30 years. He was kind enough to allow me to publish and use the data in my wateshed study (Table 2). The rainfall gauge is located along the creek which is southwest of the center of the watershed at an elevation of 152 m.

Year
Precip (cm)
Year
Precip (cm)
Year
Precip (cm)
Year
Precip (cm)
1975-76
33.8
1983-84
60.5
1991-92
62.2
1999-2000
90.4
1976-77
30.2
1984-85
59.7
1992-93
95.0
2000-01
78.9
1977-78
103.6
1985-86
96.5
1993-94
51.3
2001-02
92.5
1978-79
60.7
1986-87
41.7
1994-95
121.9
2002-03
88.1
1979-80
82.8
1987-88
45.2
1995-96
106.4
2003-04
79.2
1980-81
39.1
1988-89
49.0
1996-97
96.5
2004-05
129.0
1981-82
106.2
1989-90
48.3
1997-98
144.3
Average
78.1
1982-83
118.6
1990-91
43.7
1998-99
86.9
Table 2: Annual Precipiation in Sanders Valley Watershed
Data collected by V. Gunther

Graphic Representation of Annual Precipitation

The bar graph below (Graphic 1) is a picture of the annual rainfall data. Studying the graph reveals a few years that were significantly lower or higher than the average (78.1 cm). The first two years, 1975-76 and 1976-77, were approximately 40% of normal rainfall. These were drought years for the watershed as well as Northern California. In 1997-98 rainfall was 180% of normal. That was an El Niño year for California. El Niño arrives every 4 to 7 years with warm air masses heavily laden with moisture (Goldstein, 1999). Flooding occured throughout much of Northern California but fortunately not in this watershed.

The graph illuminates a distinct pattern. Keep in mind that the average for the 30 year period was 78.1 cm. Of the first fifteen years only five years exceeded the average precipitation. Eight of the first fifteen years received less than sixty cm of rainfall. Conversely, average rainfall was exceeded 12 times in the last 13 years. The current year, 2005-2006, though not complete is nearing 130 cm. The overall pattern for the thirty year period is of increasing rainfall. Perhaps one effect of the additional rainfall will be the expansion of the woodland areas within the watershed.

Graphic 1: Annual Precipitation in Sanders Valley Watershed

Summary

The Sanders Valley watershed is located within the Town of Moraga. It is a small watershed within the greater Moraga area watershed. The natural surface area of the watershed had been modified by typical suburban development. For the last decade the watershed has received above average precipitation which may influence the vegetation on the surrounding hills. Areas of further study would be determining rates of percolation, the depth of the water table and the chemicals present in runoff that flows into San Leandro reservoir.


References