Potential for Future Studies


Refinement of the SSC-to-turbidity-and-streamflow relations at the Gresham and Milwaukie stations will aid future calculations of SSL. Streamflow, SSC, and turbidity data are scheduled to be collected through at least water year 2012. The additional 2 years of collection can be used to refine the current regression models (especially at the highest streamflows, when most sediment is transported) and to investigate possible temporal changes in the SSC-to-turbidity-and-streamflow relation.


Additionally, continuous streamflow and turbidity data, and periodic SSC data are being collected in Kelley Creek, which is the largest tributary of Johnson Creek. An SSC-to-turbidity or SSC-to-turbidity-and-streamflow relation at Kelley Creek will provide a useful tool for quantifying how much suspended sediment originates from this area of the watershed. Kelley Creek enters Johnson Creek just upstream of the Sycamore gaging station and represents 9 percent of the total drainage area of the watershed.


To evaluate the source of sediment upstream of the Gresham station, periodic measurements of streamflow, turbidity, and SSC are being collected at two additional sites on the main stem of Johnson Creek and on another of the largest tributaries (Sunshine Creek). All three sites (not shown in fig. 1) are upstream of the Gresham station. Although these locations will not have a continuous time series of streamflow or turbidity needed to compute a daily SSL time series, comparing these measurements to values obtained at the continuous stations will provide insight into quantifying where, when, and how much suspended sediment originates in the upstream area of the watershed.


With measurements of continuous turbidity and streamflow at these three stations, monthly SSL could be computed to enhance the understanding of the suspended-sediment budget in the watershed. Installation of a turbidity probe and (or) stage sensor (along with streamflow measurements over the range of the hydrograph) at any of the three upstream sites would allow for the calculation of annual turbidity and (or) streamflow time series. Provided an adequate SSC-to-turbidity-and-streamflow relation exists, subsequent SSL time series could be computed. This calculation would result in a more precise and accurate quantification of the SSL upstream from any of these stations, rather than general inferences made from a limited number of periodic measurements.


Other measurement stations also might prove beneficial. The other substantial tributaries (North Fork Johnson Creek, Badger Creek, Sunshine Creek, Hogan Creek, and Butler Creek) account for almost 20 percent of the total drainage area of the watershed. Even periodic measurements of SSC, turbidity, and streamflow, could provide general inferences regarding the timing and sourcing of suspended sediment. 


SSC samples or turbidity measurements also could be taken at or near current or future restoration sites. Such data could provide an understanding of the effects of current or future measures to reduce suspended sediment in the stream. Evaluating and quantifying the effects of such ongoing work may help guide choices in future restoration work within the Johnson Creek watershed.


The relation between SSC and acoustic Doppler current profiler (ADCP) data in estuary or large riverine environments has been investigated in recent studies (Thevenot and Kraus, 1993; Topping and others, 2006; Wood and Gartner, 2010). The collection of acoustic Doppler backscatter data as surrogates for SSC has several potential advantages over the use of a turbidity probe. When deployed in a side-looking position, ADCPs collect data in a conic beam rather than at an individual point, and thus are able to better represent suspended sediment in stream cross sections that are not always well mixed than methods that use a point source. ADCPs are less prone to biological fouling, and as such may require less service time and fewer resources to maintain. If a multifrequency ADCP, or two ADCPs of different frequency, are deployed, sediment size can be evaluated. 


Pesticide concentration in the Johnson Creek basin is highly correlated with SSL (Tanner and Lee, 2004). Deployment of a semipermeable membrane device (Alvarez, 2010) at either the Gresham station or the Milwaukie station could be used to estimate levels of dissolved lipophilic toxic chemicals, such as the pesticide DDT, that accumulate in aquatic organisms in Johnson Creek, and often at levels less than detection limits using conventional water-sampling techniques. Provided there is a reliable relation between SSL and lipophilic toxic chemicals, a continued collection of SSL data could provide valuable insight into pesticide loading in the watershed, and could be compared with previous work in Johnson Creek (McCarthy and Gale, 1999). 


Similarly, a significant percentage of SSL (59 percent) appears to originate downstream of the Gresham station, an area of the drainage basin that includes more industrial and urban areas than the area upstream of the Gresham station. Sediment originating in the downstream areas of the watershed might contain a different suite of contaminants than sediment originating upstream in more rural areas. A contaminant study of the lower watershed would help identify potential areas of concern, such as trace elements or hydrocarbons.


First posted October 3, 2012