What a thermometer in a shaded creek is actually telling you

The question behind the question

What people usually mean when they ask about shade is: did the money and the Saturdays do anything. It is a fair question and it deserves a better answer than a photograph of some healthy alder.

The honest version is that a creek is a system with a lot of hands on it. Shade is one hand. So is the amount of water coming out of the hillside in July, and the depth of the pools, and what the neighbours upstream are doing. A thermometer reads all of them at once and reports a single number.

What the record actually shows

Our longest record comes from a reach that was fenced and planted before most of the current volunteers were involved. Read against an unshaded reach half a mile downstream, the pattern is consistent: the planted reach warms later in the day, peaks lower, and cools off faster after dark.

The interesting part is not the peak. It is how long the water spends above the temperature a young fish can tolerate.

That last measure — hours above a threshold rather than a daily maximum — is what we now report, because it is closer to what the fish experience and further from what makes a good headline.

Where the record stays quiet

• It cannot separate shade from the cold seep that enters the reach halfway down.
• It cannot tell us what the creek would have done without us, only what a nearby reach did.
• It cannot say anything at all about the years before somebody started writing numbers in a notebook.

All three of those are arguments for keeping the record going rather than for abandoning it. A short record is an anecdote. A long one is an argument.

What you can do with this

If you have water on your property and any interest in what it is doing, we will lend you a logger and show you how to tie it in. It takes an hour a month and, in about a decade, it becomes the most persuasive thing in the room.