Amount of Water Discharge in the Zanja near Mill Creek, Mentone, California

I. Introduction

The Zanja roughly translates to “ditch” in Spanish and was built in the early 1800’s by Native Americans, under the guidance of Spanish missionaries, to bring water to an outpost of Mission San Gabriel. [1]

My parents own a house along the Zanja in Mentone. Water flows at roughly bank full depth year round. The Zanja has been the subject of a number of lawsuits between various municipalities and the home owners who live along its banks. These lawsuits have focused on the issue of water rights since the county and various cities want to divert the flow of the Zanja and use it for drinking water, effectively cutting off the flow of the Zanja to the homeowners. A settlement was eventually reached in which both sides agreed not to use the water for drinking or irrigation, and the Zanja would continue to be allowed to flow through the private properties of homeowners who lived along its banks.

Many of these lawsuits happened when I was fairly young, so I don’t remember many details about them, or the studies both sides presented for their cases. Regardless of this, I was curious to see how much water flows through the Zanja. Was the amount of water that the cities wanted to get their hands on that significant? Thanks to reading the book Cadillac Desert and recently finishing a geomorphology class, my curiosity got the best of me. So I set out to find just how much water is flowing through the Zanja.

II. Methods

In order to determine the amount of water flowing through a given spot in the Zanja at any one second, I needed to find 3 variables: Depth (D), Width (W) and Water Velocity. The depth was easily determined by simply measuring across a specific spot, which we’ll call cross section ‘A’. Depth was determined by taking a series of 3 measurements across cross section ‘A’ and then averaging them. Velocity was probably the most difficult aspect. I measured the distance between two points along the bank and then threw a tennis ball in the water, recording the amount of time (T) it took for the ball to move between those two points (H). I repeated this process six times and then came up with the average time it took for the tennis ball to cover that distance.

Once I had the physical data, I did some calculations to come up with a cross sectional area of the water at that point (W x Davg) as well as the Water Velocity (H/T). The calculations for cross section was in inches and I wanted feet. Since W x Davg gives units in terms of square inches, I divided by 1 square foot (144 inches) to convert to square feet. Water Velocity was already measured in terms of feet per second, so no conversions were necessary.

III. Results

Davg = Average depth
W = Width of stream
H = Distance between two points along river
Vavg = Velocity of tennis ball averaged over 6 trials
A = Area of Cross Section ‘A’
Qw = Amount of water discharge

Davg = 6.3 inches
W = 89 inches
H = 7 feet
Vavg = .97 ft/sec
A = ( Davg x W) = 561 sq. inches / 144 sq. inches = 3.9 sq. feet
Qw = A x Vavg = 3.9 sq. feet x .97 feet per second = 3.8 cubic feet per second

IV. Discussion

My final result, after rounding to the correct amount of significant figures was 3.8 cubic feet per second. Comparing this to the discharge of many famous rivers, this amount is extraordinarily miniscule. The Mississippi River has an average discharge of 470,000 cubic feet per second. [2] The Santa Ana River, which flows to the west of the Zanja, and where much of its water ultimately ends up, has a mean annual discharge of 33.8 cubic feet per second. [3] For being one of the largest rivers in Southern California, this is a very small amount. Needless to say, we do live in a very arid environment.

Does enough water flow through the Zanja to justify local municipalities trying to take it? To simplify things when dealing with quantities of water, many organizations speak in terms of acre-feet. An acre-foot is the amount of water a family of four will need for one year. [4] According to Google, 1 acre-foot is equivalent to 43,560 cubic feet. Dividing this by 3.8 cubic feet per second, we find that it takes 11,463 seconds (or just over 3 hours) to fill the amount of space required by one acre-foot of water.

According to the 2000 census, the nearby city of Redlands has a population of 63,591 people. To simplify calculations, I divided by 4 to come up with the number of “families” who will be needing water, or the number of acre-feet that Redlands would need. Almost 16,000 acre-feet! Multiplying that by 3 hours per acre-foot, it would take nearly 5 and a half years to store enough water from the Zanja to supply the residential needs of Redlands for one year. As you can see, that in itself isn’t too practical. Not accounting for evaporation or infiltration, by itself the Zanja would be able to meet about 20% of the residential needs for the city of Redlands. This isn’t that much in the scheme of things and almost doesn’t justify the cost and effort that would be needed to bring the water into Redlands or any other city. However, in Southern California, water is nearly more valuable than gold.

V. Conclusions

My data should be taken with a grain of salt as most of the data is based on rough estimates and many assumptions. There are quite a few sources of error, such as average velocity. In most cases, you would measure velocity just below the surface, where water is flowing the fastest, as well as taking a variety of discharge measurements for multiple locations and averaging those to get an overall discharge for the river. My data represents the amount of discharge at a single spot on the Zanja and I would assume it is roughly average, based on my observations of the water level over the years. However, I have no data to quantify that.

Regardless of these issues, the amount of water flowing through the Zanja at any given moment is quite small. Given the scarcity of water in Southern California, the cost and consequences of removing the water from its “natural” channel to use for drinking water outweigh the cost of leaving the water in the channel for many to enjoy, as it runs through Redlands and many of its parks.

VI. References

[1] How big where their footprints? “Mission Era 1,” [online]:  [Accessed 30th May, 2004].

[2] LA Coast. “Mississippi River Delta Basin,” [online]: [Accessed 30th May, 2004].

[3] 1999 California Hydrologic Data Report. “11051500 SANTA ANA RIVER NEAR MENTONE, CA,” [online] [Accessed 30th May, 2004].

[4] National Resources Defense Council. “Drawdown – Groundwater Mining on Black Mesa,” [online] [Accessed 30th May, 2004].

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