Cavan riverfly
©️ Sarah Pledge

Wellies, water and wildlife: Riverfly monitoring on the Darent

Each month since October 2024 we have been part of a team of Kent Wildlife Trust volunteers monitoring the diversity and abundance of a select groups of invertebrates (mayfly larvae, caddis fly larvae and freshwater shrimp on the River Darent). 

The reason we do this is that the larvae are the rivers equivalent of a ‘canary in a coal mine’, if their numbers drop unexpectedly or they disappear completely we will know that there must have been a pollution incident. To monitor riverfly we have to do what is called kick sampling, which essentially means getting into the river with robust wellies and disturbing the bed of the river in small areas and letting the riverfly float into a net. We then put the sample into a tray for sorting into different species. We combine riverfly monitoring with testing the water for turbidity (clarity) and nitrate and phosphate levels too (part of the RiverSearch project).

When we say we monitor riverfly at Park Farm we actually get into the river at Brasted Recreation Ground because the lovely pedigree Jersey cows on Park Farm are so inquisitive that we cannot possibly set up our kit and get in the river without their very close attention….besides, the recreation ground has some useful benches and tables on which we can set up our monitoring station. This is also a lovely place to get to work as very often dog walkers and families visiting the recreation ground are interested to see what we are doing, and it is a chance to show the amazing wildlife that lives under the water. 

Caseless Caddisly Larvae

Caseless Caddisly Larvae

Ephemeridae Mayfly

Ephemeridae Mayfly

Here are our thoughts about recent sessions of riverfly monitoring: 

 

January 

So much rain! We were wondering if the water level would be low enough for us to get into the river with our net. Interestingly the Darent is a chalk stream and the water level in chalk streams remain pretty consistent (at our end of the river at any rate). The water is pretty clear and just under top of the wellies height. The temperature is mild, slight leak in one wellie but so excited about what we might discover. An abundance of mayfly larvae from the tiniest ones that would sit on a pin head to some that will soon be ready to emerge as adult mayflies. 

February 

Hmmm more rain this month but still the water level is just about okay (still below the top of the wellies!) and we have glorious sunshine for our survey. Good numbers of Brown Olives and some Blue Winged Olive larvae, lots of flat Bodied Stone Clingers - love these - we lift up larger rocks and swish them into the net. Also, lots of teeny tiny weighted caddis larvae, we have to look carefully at small conglomerations of sand and grit to see whether they start walking, then we know we have a weighted caddis larvae. 

March 

Buzzards mewling, chiffchaffs “chiff-chaffing”, spring sunshine and warm weather…woohoo! The river bed has changed again, there are now some deeper pockets, more sand than silt and shallow gravel beds forming. We also see pieces of water crowfoot float downstream. There was none when we started but the guys at the South East River Trust have been improving the flow of the river by creating leaky dams, bank re-profiling, adding pebble cattle crossing points and planting water crowfoot, water mint and water forget-me-not. This is already making the river a much more dynamic and biologically diverse habitat. Can’t wait to see what April brings. 

Learn more in our blog post about chalk streams...

Chalk streams are an ecologically significant freshwater habitat and are globally rare. England holds approximately 85% of the global total with the majority of those dotted around the south, including in Kent.

Read more here