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Home arrow Conservation arrow Ridley Temperatures Summer 2009
Ridley Temperatures Summer 2009 Print
Written by Greg Shaffer   
Saturday, 29 August 2009

On June 28th Steve Capoferri and I installed two temperature loggers Ridley Creek. I spot-checked the data on one of them a couple of times to make sure it was working, but today I read the data from both of them for the first time.  I expected their readings to be similar, but that was not the case. The differences between them are not huge, but it would definitely be better to have the high temperatures recorded by one logger than those that were recorded by the other one.

The two temperature loggers were installed about 25 feet apart and it was expected that they would record about the same temperatures. They didn't:

ridley temps 6-28 to 8-29-2009

It is difficult to see the details, but if you click on it, the full image will open in the new window (you still may need to zoom in to see the detail). 

The first thing you may notice is that by either measure, the water temperature varies quite a bit throughout the day. Of course, how much it varies depends on which logger you look at. If you look closely, you will notice that logger #1 shows some fairly high temperatures (for a trout stream) and logger #2 shows temperatures that aren't great, but are much better. The difference between the two loggers is typically between 1 and 2 degrees, although in August it was as high as 5 degrees. The simplest explanation for the difference between them is that one of the loggers is defective. That is possible, but perhaps unlikely. They are high quality instruments which are factory calibrated to be accurate within +/ 0.36 degrees (F). You may also notice that although the differences tend to be greatest during August, they vary differentially from day to day throughout the summer. If the loggers were simply defective, there would probably be either a constant difference or more clearly erratic differences.  It seems more likely that they are tracking real differences in temperature, but what could be causing them?

Spring Seeps?

My first thought was that maybe there are spring seeps in the creek. Before we started working on the deflector above the bridge last Friday (8/21/2009), I saw rising fish and people catching them on dry flies (beetles); however, according to logger #1, the stream temperature at that time was 74.3 degrees. I found information on the PA Fish Commission web site which suggested that at those temperatures, trout are unlikely to feed. I thought there might be some spring seeps in those pools, so today I went out and looked for them with a hand-held waterproof temperature probe (it stores the lowest temperature it records). I started at the deflector upstream from the bridge pool project and covered the area downstream almost to the lower parking lot. Although it wasn't a hot, sunny day, I was surprised to find the same temperature in the shallow sections and in the deep holes - 68.9 degrees. That was also the same temperature that logger #1 had recorded slightly earlier.

I am not saying that there aren't any spring seeps in the creek - there probably are, but I couldn't find any in that stretch which were significant enough to create a measurably cooler pocket of water. I measured the feeder creek below the bridge and that is a cool 65 degrees. Unfortunately, it doesn't have enough volume (at least not today) to have any measurable impact on the pool immediately downstream of it. By the way, while I was taking these measurements, I saw more than a few trout, some of which were fairly large. The bigger ones looked a little too skinny to my eye, but there are definitely still more than a few trout in the creek.

Perhaps just one explains our data

The only place where I found a measurable difference in temperature was right where logger #2 was located.  It was 0.2 degrees cooler. Just a couple of feet downstream, in a very similar area (same flow, same depth, same cover, same amount of rock), the temperature was exactly the same as everywhere else. The temperature I measured was 1.1 degrees warmer than what the logger recorded.  I may be able to explain that though. The logger is in a fairly protected area under a rock overhang.  It is pinned as close as possible to a sandy bottom and since it was installed, it had become partially buried. It may be that if there is a spring seep, the logger is basically sitting directly on cool water that is being filtered up through the sand.

ridley temps 8-16 to 8-23-2009


In the plot above, you can see that the temperatures recorded by the two loggers track one another fairly well for the first 3 days, and then diverge for most of the next 3 days, before converging again. The air temperatures were similar for those first 6 days - high temperatures between 90 and 95 degrees; however, on the 18th 0.3 inches of rain were recorded (after 8 days of virtually no rain).  This is pure speculation, but perhaps that rain filtered down and recharged the spring for a couple of days. Of course later in the day on 8/22 there were .62 inches of rain and on 8/23 there were 2.73 inches (via Hurricane Bill).  The stream was very high and muddy, so the small spring probably had no real effect on the temperature that was recorded (or it was gushing uncooled rainwater). Four days after the rain, the temperature difference between the loggers increased again, although only to a maximum of about 3.3 degrees (as opposed to the 5 degree difference recorded on 8/20).

Only a guess

Clearly I am speculating, but it seems plausible.  I have moved logger #1 to a spot adjacent to logger #2. In a week or two I will pull them out and see what I find. If they agree, then there is clearly something going on there. If not, then one of them must be defective.

Final thought - fish in the morning!

The low temperature points in the charts occur around 9am. Plenty of trout have made it through this summer (which was relatively cool in June and July), but the temperature of the water is still in the upper end of their comfort zone (at best), so they are probably feeding more in the morning than in the evening when the water is warmer.  Of course you probably didn't need a chart to tell you to fish for trout in the morning during the dog days of summer.

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