Signal crayfish are an internationally widespread invasive species that can have important
detrimental ecological impacts. This thesis aims to determine whether signal crayfish have
the potential to also impact the physical environment in rivers. A series of experiments were
undertaken in purpose-built still-water aquaria using a laser scanner to obtain Digital
Elevation Models (DEMs) of narrowly-graded gravel surfaces before and after exposure to
crayfish. The difference between DEMs was used to quantify volumetric changes in surface
topography due to crayfish activity. Two distinct types of topographic change were identified.
The first was the construction of pits and mounds which resulted in an increase in surface
roughness and grain exposure. The second was the rearrangement of surface material caused
by crayfish brushing past grains when walking and foraging, reorientating grains and altering
friction angles. A series of 80 flume runs were undertaken to quantify alterations made by
crayfish to water-worked, as well as loose, gravel substrates at low velocity flows. Crayfish
significantly altered the structure of water-worked substrates, reversing the imbrication of
surface grains to a more random arrangement. Surfaces were entrained at a relatively high
velocity flow subsequent to crayfish activity in order to directly link topographic and
structural alterations to substrate stability. Nearly twice as many grains were mobilised from
surfaces which had been disturbed by crayfish in comparison to control surfaces that were not
exposed to crayfish. A field investigation aimed to determine the potential significance of the
geomorphic impact of crayfish in rivers. Signal crayfish were tracked through a 20 m reach of
a small, lowland alluvial river for 150 days using a Passive Integrated Transponder (PIT)
system. Crayfish were active throughout the channel, although their activity became limited
as water temperature dropped and flow stage increased. Substrate was not an important
determinant of crayfish activity at this scale. Instead, crayfish tended to be found along the
inner bank of a meander bend where there was a substantial cover of macrophytes.
Consequently, signal crayfish were active for extended periods on substrates of a similar size
to those that they could disturb in flume experiments. These results suggest that signal
crayfish could have important geomorphic effects in rivers, disturbing bed structures and
increasing the mobility of coarse material. This may have important implications for both the
management of some rivers and benthic organisms that reside on the river bed.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.