loads of Larvae in my Stream

Hello,

I have recently noticed very large amounts of larvae attached to the rocks, leaves & any other submerged objects in my stream. the stream forms part of a closed system where the water from the filter outlet of my Koi pond is fed to the top of my sloping garden then runs back down to the pond in a man made stream. I have seen recently that where the stream is at it narrowest & therefore the flow is at its fastest, there seems to be a very large number of larvae attached to every available object. I have tried to include an image, are these blackfly larvae, & if so should I be concerned (my wife says I should switch the system off for  day or two & let it dry out to get rid of them)