Amanda Krull and Rada Drca


Turbidity in Response to Steam Flow Rate



Water turbidity in streams, lakes and rivers indicate poor filtration and more contaminants (Juranek and Mackenzie, 1998).  Turbidity is the Secchi depth or how far you can see the Secchi disk in the water.  Therefore, we hypothesized that the stream flow rate would positively correlate with decreased turbidity.  We found that convenience sampling was necessary due to decreased depth in particular areas limiting our sample size.  Our results indicated that increased stream flow rate has a very weak correlation with decreased turbidity (R= 0.044). 



            We hypothesized that increased stream flow rate will positively correlate with a decreased turbidity (visible depth) of the water in the Milwaukee River.  We chose the Milwaukee River because it is known to be filthy and responsible for the Cryptosporidium parvum outbreak in the 1993 (Osewe et al, 1996).  We speculated that an increase in stream flow rate would move around any filth, therefore decreasing the turbidity. 

We found information that in 1993, the pollutants that cause Cryptosporidium parvum contaminated the Milwaukee River.  The increase in stomach illness outbreaks directly correlated with a decrease in the drinking water turbidity (Juranek and Mackenzie, 1998).  We wanted to apply this correlation in connection to the stream flow rate.  The research indicates that lower turbidity in treated water would indicate successful filtering (Juranek and Mackenzie, 1998). 


Methods and Materials

            We choose a location at Lincoln Avenue and 1st Street to begin our experiment.  Initially, we planned to measure every 10 meters heading east to get new samples, but due to low depths that interfered with this random sampling we did a convenience sample by choosing areas with higher depth. Using a Secchi disk (R3), we measured the depth into the water which it disappeared and the depth in which it appeared and subtracted the appearing measure from disappearing to get turbidity (Depth appearing-Depth disappearing=turbidity).  To get this difference of disappearance and appearance of the Secchi disk we used a 50 meter Keson tape (OTR50MM) and marked the disappearing and reappearing points with tape where the disk entered and left the water to get these depths.  After measuring turbidity we used the stream flow rate meter (Geopacks unit 5) to get the stream flow rates of the eight samples collected.  We noted the locations of our samples with a compass (A-1000 Suunto).  We gathered the data on an excel spreadsheet and performed a correlation analysis.




            Our results indicated that our hypothesis was not supported.  The correlation analysis had a R value of .044 (Fig. 1).  Therefore, we found that the increased stream flow rate has a weak correlation with decreased turbidity.  Our data had a very limited range the Secchi depth ranged from 0.2m-0.8m and Stream flow ranged from 4m/s-8m/s.


Fig.1 Turbidity in Response to Stream Flow Rate (SFR=stream flow rate, Exponential (SFR)= exponential stream flow rate)



            When planning our sampling, we initially wanted to do random sampling.  We found research supporting our hypothesis that turbulence in water is responsible for suspension of debris and minerals, which in turn lowers turbidity (Leighly, 1934).  Due to depth issues of the river in our location and the accessibility of the locations, we were limited to conducting convenience sampling.  This left us with a sample size much smaller than initially planned.  The location in which we ended up sampling was poorly maintained; therefore the turbidity in this area may have been affected.  If good filtration did correlate with turbidity, some areas of this water may not have been filtered as well (Juranek and Mackenzie, 1998).  Filtration affects turbidity by increasing turbidity in poorly filtered areas, which is why we searched for a connection between the stream flow rate and turbidity. 

            If we choose to sample again we would probably select a few convenience samples from various locations of the river to get better results.  We also think that comparing the filtrate in the sampling areas to see if our turbidity correlates with the filtration rate may help us to find a stronger correlation.  There are other factors that may have influenced the turbidity. Strong winds could make a faster stream flow rate. Decreased sunlight could impair visibility.






Juranek, D.D., Mackenzie, W.R. (May 1998).  Drinking Water Turbidity and Gastrointestinal Illness.  Epidemiology, Vol. 9, 3, 228-231.  Retrieved on 7  October, 2007 from: Jstor


Leighly, J.  (1934).  Turbulance and Transportation of Rock Debris by Streams. Geographical Review Vol. 24, No 23 pp. 453-464.  Retrieved on 2 November 2007 from: Jstor


Osewe, al.  (1996).  Cryptosporidiosis in Wisconsin: A Case Control Study of Post Outbreak Transmission.  Epidemiology and Infection Vol. 117 No.2 pp.297-304.  Retrieved 26 November 2007 from: Jstor