Constructing Riffles

For a number of years, elevated iron bacteria levels had been observed to originate in Bilston creek around Lippincott bridge.  This causes an orange spongy deposit on the streambed and plants.  While the bacteria is not directly harmful to fish, it interferes with and reduces the growth of feed organisms as well as being unsightly. 

A series of two riffles were designed to raise the level of holding water upstream by a few inches which hopefully would reduce the amount of iron-rich groundwater from seeping into the stream.  Water flowing over the riffle structure also increases the amount of oxygen in the water. 

In the Firehall Creek tributary, three riffles were constructed to increase the oxygen.  Although this particular section of Firehall Creek dries up in the summer, it is a possible spawning area during the spring, after which the fish would migrate downstream.  An increase in the oxygen resulting from the riffles will also assist in the breakdown of spongy deposits.

The project was undertaken at a time when the water level was very low so no bypass flow pipe or channel was needed.  The streambed was excavated about 30 cm to reach a stable base. An excavator was used as it greatly reduces the amount of damage that could occur to the streambed from many trampling feet.  Cobbles selected for this project were hand-picked to be rounded glacial till as would be found in a natural streambed. 

This is a concept drawing of the constructed riffle structure built in Bilston creek at Lippincott bridge.