St. Marks River Discharge Measurements September 2017

Methodology

Introduction

The Riversurveyor M9 ADP is designed to measure

River discharge
3D water currents
Depths
Bathymetry

Prior to starting a flow measurement there are three pre-tests that are required

Set time
System test - tests to see that all cables are connected
Compass Calibration

After the pre-tests are completed flow measurements are obtained through a cross section of the stream (transect). The total discharge is calculated by the cross-sectional area times the mean water velocity.

The ADP is positioned in a boat (foam boogie board) and placed into the stream to start a transect. The boat is located at the start edge of the transect. This is where the software shows a depth of two cells (approximately 18 inches). Next, the start measurement button is selected and the boat is moved at a constant rate of speed through the use of a pulley system that is set up across the stream and perpendicular to its flow. When a depth of two cells is reached at the end edge (the opposing river bank) the measurement is stopped. Another transect is then completed by moving that boat back to the bank at which it started. This represents one discharge flow measurement. The FGS will follow the USGS protocol which calls for three measurements. These values are then averaged to determine the discharge for the stream.

Site map showing location of discharge measurement sites

Site Locations

The area of interest is the St. Marks River above the river rise. The yellow points depicted on the site map represent the locations for discharge measurements. These locations are Horn spring, Natural Bridge spring, and St. Marks River downstream of all the Horn springs (there are several small springs near the main Horn spring).

Horn Spring

Horn spring is a second magnitude spring that contains an abundance of suspended particles giving the water a greenish hue. It also contains large quantities of exotic vegetation, most notably hydrilla. The hydrilla along with numerous trees and logs form a dam along the spring run. Water flowing over and through this vegetation is channelized and doesn’t provide an even rate of flow through the spring run channel. Since this affects the discharge measurement, a six foot section of hydrilla is removed from the spring run prior to a measurement. On September 26. 2017 it was further decide to remove as much hydrilla as posiible to avoid the channeling of water through the vegetation. This seemed to work and the flow rate along the transect was more evenly distributed. A variance error of 7 percent for the three measurements taken was the smallest received over the three previous visits. An ideal variance error is 5 percent or less. A complete clean out of logs and hydrilla up to the spring pool may provide more consistent measurements in the future.

Natural Bridge Spring

Natural Bridge spring is a first magnitude spring located just east of the Natural Bridge monument. The water is clear to slightly tannic and the run contains many submerged logs. Great effort is taken to avoid transects near submerged logs, however they are impossible to avoid and numerous roots along the edge of the run will have some effect on our measurements. Even with these impediments, consistent reading have been obtained from this location.

St. Marks River Downstream

St. Marks River downstream from Horn spring is an ideal location to measure dicharge in the St. Marks River. The river is approximately knee deep and there is no vegetation or submerged logs. The difference between maximum and minimum measurements was only 2 percent resulting in highly consistent discharge values. During next quarters visit it is likely that a new station will be added (St. Marks River upstream of Horn spring).

Discharge measurement transect St. Marks River Downstream

Historic Discharge Measurement Data

Limited discharge data has been obtained for this area. Natural Bridge spring has discharge measurements dating back to 1942 but only 8 measurements have been taken as of September 26, 2017. Horn spring and downstream of Horn spring on the St. Marks have even less data. The need exists to gather more data for the upper St. Marks River in order to understand the hydrogeology as well as the Wakulla Basin.

Just below the river rise the USGS has obtained daily discharge readings dating back to 1956. After gathering more upstream data it may be possible to estimate historic (and future) flow based on the correlation with USGS daily measurements taken at the river rise. Below are tables and plots of the Historic data taken at the three locations.

**Historic Average Discharge (cfs) Measurements**
SiteID	Date	StationName	AvgDischarge
HORN	1971-11-12	Horn Spring	28.80
HORN	2002-02-20	Horn Spring	14.15
HORN	2016-09-30	Horn Spring	14.47
HORN	2017-06-12	Horn Spring	15.89
HORN	2017-09-26	Horn Spring	19.81
NATBS	1942-05-19	Natural Bridge Spring	115.00
NATBS	1946-05-14	Natural Bridge Spring	132.00
NATBS	1960-12-05	Natural Bridge Spring	97.00
NATBS	1963-05-15	Natural Bridge Spring	79.00
NATBS	1971-10-06	Natural Bridge Spring	106.00
NATBS	2002-04-25	Natural Bridge Spring	151.98
NATBS	2016-09-30	Natural Bridge Spring	151.32
NATBS	2017-09-26	Natural Bridge Spring	141.03
SMDS	2017-09-26	St. Marks River Downstream	56.73

Current Discharge Measurement Data

The table below shows the discharge measurements taken durning the latest visit. All data obtained by FGS staff using the M9 Riversurveyor can be found in the geodatabase located here: *P:401601-Cave_Karst_Database

**Latest Average Discharge (cfs) Measurements**
SiteID	Date	StationName	AvgDischarge
HORN	2017-09-26	Horn Spring	19.81
NATBS	2017-09-26	Natural Bridge Spring	141.03
SMDS	2017-09-26	St. Marks River Downstream	56.73

Below are plots of discharge measurements taken by the Florida Geological Survey at the three locations since September 2016 and daily discharge and stage measurements taken by the US Geological Survey above and below the river rise since September 2016.