King County Large Lakes Conductance Evaluation
In the past several months, the conductance profiles in lakes Sammamish, Washington, and Union have been higher than has typically been observed (~1993-2022). Instrument checks by the King County Environmental Laboratory indicate that the instruments have been accurately calibrated. Below, I explore historical trends in lake conductance and alkalinity and I evaluate trends in a number of tributary streams as a preliminary investigation into the cause(s) of the recently observed increases.
Central Lake Washington and Lake Sammamish trends
First I explored the data a bit and found that because of salinity intrusion into Lake Union, I couldn’t easily discern anomalous conductance values in Lake Union or at the Locks. Therefore, I first plotted surface conductance and alkalinity at the central Lake Sammamish (0612) and Lake Washington (0852) stations and at Station 0540 in the Montlake Cut.
The conductance spike in 2014-2015 at Montlake (0540) obscures the recent spike in conductance, but the spike is clearly evident in surface conductance in Lake Sammamish (0612), which reached a level in 2023 not observed in the records going back to 1993. The recent increase in conductance in Lake Washington (0852) is evident in the plot below, but higher surface water conductance was recorded 1993-1999.
We no longer measure alkalinity at Montlake (0540) and the recent alkalinity in Lake Sammamish and Lake Washington is within the range of historical variability.
I then calculated whole-lake volume-weighted means of conductance and alkalinity at the central Lake Sammamish and Lake Washington stations and plotted the trends from 1993 to present. The recent rapid increase in conductance is evident in the plots, while the recent alkalinity increase is not as dramatic.
Influent tributary station trends
I selected some long-term influent tributary monitoring stations and plotted trends in conductance and alkalinity (1998-2023). With the possible exception of Juanita Creek, there does not appear to have been a sharp increase in influent tributary conductance or alkalinity.
Let’s focus on shorter records
To better visualize any recent increases/spikes in conductance and/or alkalinity, I landed on plotting data starting in 2010. ….Maybe see higher peaks in 2023 in other creeks…trend toward higher winter minimums? Anyone want to help figure this out?
How about the trend in a simple annual (Water Year) average? We now have all of the 2023 Water Year data in hand (Streams are up to date in through September 2023 in Socrata as of 11/15/2023).
How about the trend in a simple winter (Nov-Mar) average?
In general, I think I a tendency of a recent increase in conductance in several tributaries, including the Cedar which contributes very roughly 60% of the total inflow to Lake Washington.The winter conductance in Issaquah Creek (about 70% of the inflow to Lake Sammamish) also rose quite a bit between winter 2022 and 2023.
Winter (and annual) alkalinity patterns were a bit more mixed. Overall, it looks like tributary inflow increased in conductance might be a plausible explanation for the observed increases in our largest lakes. I’ve planned on setting up a simple mass balance (box) model of the lakes to more directly evaluate this hypothesis but I haven’t worked that up yet.
If confirmed (i.e., tributary conductance has risen rather sharply since last Water Year) resulting in an increase in lake conductance, then one might wonder why conductance has increased. It doesn’t seem like we had a lot of snow (and associated salt applications) this past winter. Could it be some change in surface-subsurface hydrologic interaction?
Also, the long-term trend in increasing conductance (and alkalinity) in King County lakes and streams has been reported since as early at least the 1970s (for example see this this 2020 lake report starting on pdf page 87 and this 2023 stream report starting on pdf page 51).
). Do these increases have any ecological effects? If so, is there something we should be measuring that could demonstrate them?
Just rambling here….again, if anyone is interested in helping figure this out….the data I’m working with are publicly available here: Water Quality | King County | Open Data
One possibility is that the drought conditions that we seem to be experiencing more regularly these past few years might be an explantion for increasing conductance in local streams and rivers…less precipitation resulting in longer groundwater residence times and longer contact with rocks and minerals…?
I’ve been working with a number of USGS river gauges (currently 11) with data going back to at least 1930 (remember the Dust Bowl?) to try and put recent flow trends into a longer perspective. I also borrowed the Puget Sound Partnership’s Low Flow Vital Sign approach….look at the number of days between July 15 and September 30 that flows go below the period of record 25th percentile for that day. Note that I changed their baseline and used the period of record rather than 1948 to 1978 so we’re basically looking at anomalies over the period of record.
Look at the first plot above. 2023 was the only year over the period of record that all 11 gauges recorded more than 75% of days between July 15 and September 30 with flows less than the 25th percentile for that day!
The second plot seems to indicate (at least to me) that total flow (or mean flow if you like) was actually lower during those Dust Bowl years…trended upward during the cool phase of the PDO and then declined and may be trending upward again…or remaining relatively constant around a mean level that is still higher than at the beginning of these records….