YSI 6200 data sondes were deployed vertically in a pool deep enough to contain water during dry conditions. Depth is also measured using a depth sonde integrated with a Sigma autosampler.

YSI: The YSI 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. This file 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). In this file they are the same.

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

SIGMA AUTOSAMPLER: Depth is also recorded at times using a depth sonde integrated with the Sigma autosampler. Sonde was secured to cinder block driven into the stream bottom and referenced to YSI depth. When available, YSI depth is used in preference to Sigma depth because the latter is less reliable. Sigma depth is used for discharge from from 2004-12-23 to 2004-12-31.Discharge using Sigma depth should be used with caution.

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 StandardizedHoboDepths

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)

Sigma Depth is used for a short period of time late in the year. To apply a single rating curve, Sigma Depths were standardized to HOBO depths accorinding to
(StandardizedHoboDepths =SIGMA_DEPTH + 0.237)

Sigma Flows are included in this file as well, but these are based on an old, less complete rating curve.