YSI 6200 data sondes were deployed vertically in a pool deep enough to contain water during dry conditions. The sonde was deployed in a PVC tube, which was attached to ree-bar driven into the stream bed. The PVC tube had holes drilled to allow adequate water circulation. Dissolved oxygen measurements are of poor quality because the the bottom of the pool was often filled in with detritus following intermediate sized storms, leading to anoxic conditions. To alleviate this problem, the PVC tube was raised 10.1 cm on 7/16/03. The file for 2001 includes the raw depth measurement (RAWYSI_Depth), as well as the depth measurement corrected to the YSI depth after the change in tube height (YSI_Depth_STD) and for which the rating curve has been standardized.

The PVC tube was surveyed upon original deployment in 2001 and resurveyed in 2002 to ensure that elevation did not change during winter ice period. No change was detected. No evidence of change has been noted through 2006 based on discharge measurements. See discussion below for development of the discharge-stage relationship.

Recalibration of oxygen, pH, conducitivty and depth was performed roughly every 4-6 weeks. Instrument drift during deployment periods was not significant.

ADDITIONAL RESEARCH LOCATION COORDINATES: In addition to the decimal degree coordinates given above, locations are also given here in Massachusetts State Plane coordinates. The stream is called SawMill Br. and is located in Burlington MA. Small amounts of forest remain (~14% in 1999), much of which is along the stream channel near the gaging site.

STATION STATE X STATE Y
YSI-SB 225840 919310

NOTES AND COMMENTS:
Dissolved oxygen concentrations generally reflect water column levels. However, due to the position of the sonde, detritus periodically covered the bottom of the sondes where the D.O. probe is located, leading to anoxic conditions. It is likely that low D.O. levels reflect benthic conditions.

Velocity measurements were made using a Marsh McBirney current meter and starting in 2009 with a Sontek Flowtracker. Discharge estimates in Saw Mill are based on a power rating curve that contains measurements up to a stage depth of 1.2 meters, which is above bankfull stage. Rating curve has an R2 of 0.89 using 40 measurements between 2001 and 2009. The rating curve has not noticeably changed over the period. Prior to 2007, stage was measured with various other instruments (YSI, Sigma depth sonde, HOBO). All files in this series (2001-2009) are now standardized to HOBO depth. The rating curve is based on the standardized HOBO depth's (and actual HOBO depths since 2007).

CALCULATIONS:
Discharge rating curve :
Discharge (l/s) = 889.65*depth^5.8788
where depth is Adj_HOBO_Depth

Several depth are included in this file to allow comparison with earlier file version. RAWYSI_Depth is the depth measured by the YSI and is used by an initial, preliminary rating curve. YSI_Depth_STD is depth adjusted to standardize for the change in stilling well depth in July 2003. The column Depth_Device is included when combining files from several years, over which several different devices were used to calculate depth. Depth_STD is the depth that is standardized across all measurement devices so that a single rating curve can be applied. The rating curve is standardized to the Hobo depth measurements, which were started in 2006. The difference between Hobo data logger depth measurements and YSI data logger depth measurements (YSI_DEPTH_STD) is 0.211m
(StandardizedHoboDepths = YSI_Depth_STD + 0.21)

Also, at times depth and discharge may be periodically erroneous due to leaf pack accumulation at a riffle downstream causing backup. We remove this whenever we visit the site. Generally we believe the depth error is on the order of 2-4 centemeters. The depth data have not been corrected for errors associated with temporary debris dams.

OLD RATING CURVE ( prior to December 2009 revision):
Discharge measurements in Saw Mill are based on a power rating curve that contains measurements up to a depth of 0.6m (using the column RAWYSI_Depth). Flows above this amount are estimated using a linear extrapolation using the six measurements with the highest flow. Thus flows determined from levels greater than 0.6m are approximate (roughly 1% of the readings in 2001 deployment period). Rating curve below 0.6m is based on a power relationship with an R2 = 0.98. Measurements are based on 12 measurements taken between 2001 and 2003.

OLD CALCULATIONS:
When stage in SawMill Br. is less than 0.6m :
Discharge (l/s) = 1245.2 (depth) 4.782

When stage in SawMill Br. is greater than 0.6m :
Discharge (l/s) = 560.7 (depth) – 229.8