Analysis of Stream Flow for the Guadalupe River

Kerrville to Spring Branch

Summer Flow Rates 1951 - 2000

 For 

Southeast Trinity Groundwater Conservation District
P. O. Box 203

Bulverde, Texas  78163

 

 By

 SGA Consulting Services
981 Brookside Dr.
Spring Branch, Texas  78070

 September 11, 2000
Revision 1 – September 21, 2000 

                                                                                  


Foreword

     This report has been prepared pro bono and independently by Stephen C. Grigory, PE, SGA Consulting Services. http://grigory.com The report has been prepared for the benefit and use of the Southeast Trinity Groundwater Conservation District. (STGCD) but is not restricted to that group.  No request was made for this report by STGCD. 

    The purpose of the analysis is to bring into perspective the effects of present day water use on the Trinity Aquifer and on the Guadalupe River. This was done by comparing the river flow in the months of July, August and September from 1951 through the present time.  The Guadalupe River typically has its lowest flow of the year during these months. 

    It is hoped that the STGCD and the general public will find the data helpful in evaluating proposals for future use of the aquifer.  Distribution of this report is unlimited.  However, it must be distributed in its entirety.  Further, the data in this report cannot be interpreted properly if it is reproduced in black and white.  Color copies are required. 

    Figures 2, 3 and 6 are intended to demonstrate the basic hydrology of the river and aquifer when certain conditions of stream flow exist.  These figures do not purport to show actual levels of either the river or the aquifer.

Revision 1 September 21, 2000:           1984 data added to Figure 5                             

 

Introduction

     Sometime before September 7, 2000 when the pictures in this report were taken, the Guadalupe River went dry at some point below the USGS stream flow station 08167500 near Spring Branch.    This gage is located on the downstream side of the Ranch Road 311 bridge.  The pictures of the dry river bed were taken at the Rebecca Creek Rd. low water crossing which is 4.5 river miles downstream from the Spring Branch gage and approximately  7 miles upstream of Canyon Lake.   

    The stream flow at the Spring Branch gage has been steadily decreasing and now stands at about 6 cubic feet per second (cfs).  This means that nearly 4 million gallons of water are disappearing in a 4.5 mile stretch of river each day.   In addition, the drop in flow between the USGS gage near Comfort and the Spring Branch gage is 11 cfs or an additional 7 million gallons per day.  

    A short time ago the TNRCC was alarmed at this trend and was actively looking for people illegally pumping from the Guadalupe river.  It turned out that the gage at Spring Branch was faulty and the loss was attributed to the bad gage.  (However, the drop in flow between Kerrville to Comfort was ignored.   The drop between Comfort and Spring Branch, although reduced by the repair, was still evident after the gage was repaired and this was also ignored.)  The investigation was dropped. 

    The reason the TNRCC got excited about the drop in flow between the upstream gages and downstream gages is that this is atypical of a spring fed river even in times of low rain fall.  The Trinity aquifer feeds the Guadalupe river along this entire stretch and the river flow increases as it moves downstream even in the absence of rain. 

    A review of the stream flow records shows that in the summer of 1996 and in the summer of 2000, the Guadalupe River, for the first time, is drying up starting at the lower elevation and proceeding to the upper elevation.  Normally, in time of drought, the Guadalupe River dries up fairly uniformly along the length of river between Comfort and Spring Branch.  In some years the Comfort gage registered zero flow first and in others the Spring Branch gage went dry first.  In any case, if the flow at one gage registered zero, the flow at the other gage was extremely low. 

    Prior to 1996 there had been no significant reduction of flow rate between gage readings at Comfort and at Spring Branch during an absence of rain.   The conclusion of this analysis is that the Guadalupe will run dry downstream of Highway 281 more frequently in the next 50 years than it did in the past 50 years because of increased use of the Trinity Aquifer.

Summer Stream Flow Analysis of the Guadalupe River 1950-2000 

Months of July, August and September for the years 1986 – 1999 

The months of July, August and September are months of maximum water usage for the Trinity aquifer and minimum stream flow for the Guadalupe river.  Stream flow data for these three months were extracted from tabulated data for the following USGS stream flow stations on the internet.

 

bullet08166200        Guadalupe River At Kerrville, Tx   Elevation 1601 ft, installed 1986
bullet08167000        Guadalupe River At Comfort, Tx   Elevation 1370 ft, installed 1939
bullet08167500        Guadalupe River Nr Spring Branch, Tx   Elevation 948 ft, installed 1922

 The data for 1986 through 1999 are plotted on the graph in Figure 1. 


Figure 1  Guadalupe Stream Flow for July, August, September  -  1986 – 1999

     Note that the downstream gage at Spring Branch (black line) is higher in most years than the upstream gages at Comfort (red line) and Kerrville (green line).  This relationship between the river and the aquifer is illustrated in the simple sketch in Figure 2. 

    In the absence of rain the aquifer level drains to the creeks and rivers until the level of the aquifer is lower than the river.  At this point the river runs dry.   The aquifer water also runs downhill within the aquifer itself but this is thought to be a slow movement.  The path of least resistance is drainage via the creeks and rivers.  Of course pumping draws down the aquifer level as well. 

    The simple sketch shown in Figure 2 shows the relative levels of aquifer and river at Comfort and Spring Branch for most summers from 1986 to 1999.   The river level a Comfort is lower

Figure 2  Normal Summer Stream Flow Between Comfort and Spring Branch

 than at Spring Branch because the artesian pressure of the aquifer feeds the river and tributaries all along its length, while the level of the aquifer relative to the ground surface may not change much along the river.  The arrows indicate the direction of water movement in the aquifer.  

Then in 1996, the first really low summer flow since the 1960’s, a different phenomenon occurs.  As illustrated in Figure 3, the river flow rate decreases as it flows downstream.  The river is fed by the aquifer above Comfort but somewhere below Comfort, the aquifer level drops below the river.  The river begins to recharge the aquifer.  If we assume that there is no great amount of pumping directly from the river, then this must be caused by increased drawdown of the Trinity aquifer by pumping.   In addition, increased drawdown of the Edwards aquifer could cause increased outflow from the Trinity.

Figure 3   In 1996 The River Flow Decreases Between Comfort and Spring Branch

 Stream Flow Records for the Worst Drought of Record 1950 –1957

     To support the observations for the years 1986 through 1999, the data was scanned for years the Guadalupe went dry at the Spring Branch gage or the Comfort gage.  This happened every year during the drought of the 1950s.   The graph of the data is shown in Figure 4.


Figure 4  Stream Flow for the Worst Drought of Record 1950-1957

In this graph the entire year of stream flow is plotted for years 1951 through 1957.  Note that the flow rate at Comfort (red line) and the stream flow at Spring Branch (black line) remained very close to the same value or the stream flow at Spring Branch was much higher than at Comfort.  Also note that rains that generated flows over 150 cfs (vertical spikes in the curve) were not sufficient to restore normal flow to the river for any length of time.  

    The river ran dry at both locations about the same time and in 1953 the river ran dry at Comfort and did not quite run dry at Spring Branch.  The river was near running dry at Comfort in 1957 when a flood of 10,000 cfs at Comfort and 15,000 cfs at Spring Branch ended the dry year.  As will be shown the drought was not really ended by this flood. 

    The point of this data is to show how the river and aquifer behaved in dry years when there was no excessive demand and no significant rainfall. 

Stream Flow Records for Summers of 1963, 1964, 1996 and 2000

     The river went dry at both the Comfort gage and the Spring Branch gage in 1963 and 1964. The Guadalupe was running dry at Comfort and Spring Branch simultaneously in 1984 before a rain raised the flow rate to 40-cfs.  Flow was higher at Spring Branch than at Comfort or about the same value for the remainder of the summer.   In the summer of 2000 the river is visibly dry at Rebecca Creek bridge and nearly dry at Spring Branch.  These data are plotted in Figure 5 to again illustrate how the river flow has changed in the very recent past.


Figure 5  Steam Flow for July, Aug and Sept for 1963, 1964, 1984, 1996 and 2000

    Note that in both 1963 and 1964 the Guadalupe River went dry at the Comfort gage before it went dry at Spring Branch..  (The Kerrville gage was not installed until 1986).  However, in the year 2000 the river is dropping rapidly at Spring Branch while there is decent flow at Kerrville and Comfort.  (The gage at Spring Branch was out for a month and only 30 days of recent data is available for Spring Branch and Comfort.)  There is also evidence of aquifer recharge from the river in the Spring Branch area in the summer of 1996    In early summer of 1996 the relative flow rate are similar to summer of 2000 prior to significant rainfall.

 

 Figure 6 Relationship Between the Guadalupe River and the Trinity Aquifer, Summer 2000

 

Conclusions and Recommendations

 Conclusions 

bulletThe dynamics of the Trinity aquifer and the Guadalupe River have changed during the last five years during periods of low flow.
bulletThe Guadalupe river is, for the first time, recharging the Trinity aquifer from Comfort to Canyon Lake with Trinity aquifer water from upstream of the Kerrville gage.
bulletThe Guadalupe river has gone dry at Rebecca Rd Bridge while the flow rate at Spring Branch is 6 cfs and the flow rate at Comfort is 17 cfs.  This is a historical first.
bulletIf nothing is done about water use, the Guadalupe river will run dry with greater frequency in the future.  This is both aesthetically and environmentally unacceptable.  

Recommendations

bulletDetermine if water use by homes and ranches over the Trinity aquifer can account for the recharge of 6 cfs into the aquifer in less than 4.5 miles of river and 11 cfs in 60 miles. (Is drawdown of the Edwards aquifer draining the Trinity at an excessive rate?)
bulletUse the relative flow rates of the Spring Branch and Comfort stream flow gages as a signal for water use restrictions. 
bulletThe stream flow at Spring Branch should not be allowed to drop below the stream flow at Comfort in periods of drought.  The Guadalupe river should never be allowed to run dry from excessive water pumping.

 

This site is maintained by
Stephen C. Grigory, PE
981 Brookside Dr
Spring Branch, Texas 78070

email scgrigory@grigory.com for technical comment or questions
email webmasters@hillcountrywater.org concerning problems with the website.