Part of our Upper Arun River Revival project is to survey the River Arun from its source, north of Horsham, ~50km downstream to Pallingham which is the tidal extent of the river. The surveys involve walking 500m sections of river and recording where natural habitat is present for freshwater biota (e.g. fish and invertebrates). We also record the presence of non-native species and any factors that may be negatively impacting the river. This information gives us a valuable insight into how the river is functioning for the benefit of wildlife and people and helps us to identify where we can make improvements.
Our surveys started along Hyde Ghyll which is one of several headstreams arising in the Wealden clays around St. Leonards Forest that provide the Arun’s base flow. Here the water flows through undisturbed woodland before feeding into Hawkins pond where the Ghyll was dammed nearly 500 years ago to provide a source of water that powered a hammer mill. The hammers were used for processing smelted iron that was harvested from the iron ore found across the Weald.
Whilst surveying the Ghyll, I took the opportunity to sit and watch the rich invertebrate life that was whizzing around in the water beside me. I first noticed the freshwater shrimps and soon became aware that the water was teaming with cased caddis fly larvae, all clambering around on the channel bed. I was fascinated to see so many and took the opportunity to take a video of one climbing up and feeding off algae attached to a twig that had become lodged in the channel substrate. Although I am no expert, freshwater invertebrates, especially caddis flies, have always fascinated me. Adult caddisflies live a terrestrial life for about 1 month, but before that they undergo 3 life cycles. They start out as eggs which are laid by adult females on vegetation just above the waters surface. The larvae then hatch and fall into the water where they immediately start to build protective cases. These cases can be made from silken thread, or twigs, gravels and even plastic. The type of case can vary between species, by season and by habitat with species in flowing water generally building cases comprising heavier materials to increase their overall weight.
Following the larvae stage, the larvae will attach itself to a stationary object such as a stone and creates a cocoon of silk around the body where it pupates before floating to the surface to become an adult.
Caddis flies are an important group of insects. During their freshwater stages of life, they play a role of both consumers, feeding off detritus, algae and/or other insects and are also prey for a range of fish species. During their adult stage they are also an important source of food for brown trout that will leap from the water to take flying adults and are also consumed by birds and bats. Some caddisfly larvae are also sensitive to pollution and they are often used as indicators of the quality of rivers.
It is hard to imagine that the small Ghyll with crystal clear water eventually becomes the River Arun which, by comparison flows within a wide, over-deepened channel that has been modified by weirs, road bridges and culverts in the past. That is not to say, however, that the River Arun does not have valuable habitat for wildlife. Even through Horsham the river is mainly wooded along its banks. This helps to protect the riverbanks from erosion, reduces the water temperature which benefits many fish species and the trees also provide a source of woody debris in the channel. This helps to create variable bed levels and sediment bars which benefit a range of species, particularly invertebrates. Caddisfly larvae for instance benefit submerged stones which can be exposed if sediment is trapped by woody debris and from woody debris itself as both features provide the best substrate for the growth of algae and periphyton (micro-organisms) from which most larvae will feed. Fish also benefit from woody debris as it plays a role in trapping sediment which helps to expose gravels downstream that are used by fish to lay their eggs. During a survey of the river downstream of Rudgwick I was lucky enough to witness fish spawning on gravels exposed due to a large debris dam located upstream. Both species will lay ~10,000 eggs which adhere to the gravels
River Arun downstream of Horsham