Effect of Rain the Week of Sept 13, 2000 on River Flow and Recharge

By

Stephen C. Grigory
SGA Consulting Services
Spring Branch, Texas

The graph is a plot of Guadalupe River stream flow for the week of September 12 – 18, 2000.  The red line is the flow rate at the USGS gage at Comfort. The black line is the flow rate at the USGS gage at the Hwy 311 gage at Spring Branch.  It is 60 river miles between these two gages. 

Rebecca Creek Road crosses the Guadalupe River 4.5 river miles below the Spring Branch Bridge.  There has been no flow at this point on the river for some weeks.  This indicates that any flow past Hwy 311 is being recharged into the Trinity aquifer or some other repository of groundwater along this stretch.  There is a known recharge spot on the river at a geologic feature called Mueller Falls as, historically,  no visible flow is observed past this point when the river is very low.   

The graph in Figure 1 shows that a thunderstorm caused a sudden rise in the river at Comfort on the morning of Sept. 13.  No rise occurred in the river at Spring Branch until 48 hours later – a flow velocity of about 1.25 mph.   

On the evening of Sept. 16 at 7:00 pm,  I personally observed the river at Rebecca Creek Road bridge and there was absolutely no flow.  Assuming that the river started flowing the moment I turned my back, there was a 40 hour delay between the rise at Spring Branch gage and the Rebecca Creek Rd bridge 4.5 miles down stream.  This would be a velocity of 0.11 mph.  Any casual observation from Hwy 281, Hwy 311 or the Rebecca Creek Bridge will indicate that the river is flowing much faster than that.  At 1.25 mph the water should have appeared at Rebecca Creek Rd in less than 6 hours after the rise at Spring Branch or about 8:00 am on Sept. 15. 

If the water passing Hwy 311 does not go downstream to Rebecca Creek Rd. then it is either being pumped out by some bodacious pumps that were just waiting for this slug of water to come down the river or it is being recharged into the aquifer.  If we say the average flow past the Spring Branch gage was 45 cfs during this 40 hour period, then 48 million gallons disappeared in that time frame.  Put another way, recharge occurred at 20,000 gal/min  for over 40 hours.             

When the aquifer had been recharged to capacity locally it could no longer take water at that rate and water began to by-pass the recharge zone.  Flow was then observed at Rebecca Creek Rd some time between my observation at 7:00 pm on Sept 16 and a second observation on the afternoon of Sept 18.  Too bad there is no gage at Rebecca Creek Rd. 

I believe the Trinity aquifer has been drawn down by local use in a manner unprecedented in history.  A second possibility is that drawdown of the Edwards aquifer is in turn draining the Trinity at an unprecedented rate.  In either case there is a good possibility that a large cavity exists in the Mueller Falls area that can absorb water at a high rate when it is empty or the entire 4.5 miles between Spring Branch gage and Rebecca Creek Rd bridge is an excellent recharge area.   

The question remains, can the Trinity aquifer sustain any increase in population when it is being drawn down so dramatically by present use in this moderately dry period. 

The graph in Figure 2 shows stream flow data for the Spring Branch and Comfort Gages from 9/13 through 9/23/2000.    For a few days after the rain the river receives water from the aquifer but the effect is temporary.  The aquifer simply does not recover from a single downpour.  Even though the flow at Comfort is holding steady at 25 to 30 cfs, the flow at Spring Branch drops until on 9/20 the flow again drops below the flow at Comfort.  The river is once again recharging the aquifer.   

Further, the flow at Rebecca Creek Rd. bridge is at a trickle again while the flow at Spring Branch is a moderate 18 cfs.  In the absence of a recording gage at the Rebecca Creek Rd. bridge, the flow was documented photographically.  The photograph of the river just downstream of the Spring Branch gage (hwy 311) is shown in Figures 3.  The photograph in Figure 4 is of the flow at Rebecca Creek Rd. bridge.  The photographs were taken at noon on 9/23/00.

Figure 1  Large Recharge of Aquifer by Guadalupe  After Rain at Comfort

Figure 2  After 9/20 There is Steady Flow at Comfort as Guadalupe Again Recharges The Aquifer