A primer on fish passage barriers
By Jim Hearsey
PURPOSE
The purpose of this primer is to help field personnel in the identification and
classification of obstructions that may prevent passage of fish from one section of a
stream or river system to another.
METHODOLOGY
The general methodology we will use is to first locate potential obstructions and then to
assess the likelihood of fish being able to pass through these barriers. Upstream passage
is generally more difficult for fish so we will use that as the benchmark of accessibility
when looking at some obstacles. However, we will still have to verify passage in both
directions.
FINDING POTENTIAL BARRIERS
Before we can make an assessment of a particular barrier, we must locate them. There are
several ways of doing this. We could walk in the stream with a net and head upstream,
scaring the fish ahead of us and wait until they are forced back down into our net and see
what stopped their progress away from us. This would be detrimental to the stream bed
however and would be counter to our general purpose of benefiting stream fishes. This
will not be done and is illegal in many waters.
There are two methods suggested. The best is to walk along the bank of the stream for the
whole section or sections being checked. When doing this, one must try to impact the
area as little possible as the edges of streams are fragile and important ecosystems. The
other approach is to follow roads up the stream valley and make checks only where the
road crosses a creek or stream. This will be the easiest method and will capture most of
the anthropogenic (man-made) causes of passage interruption since road culverts are the
main culprit. You may still miss abandoned roads or railroad grades that may have been
out of use and grown over for decades, or longer. You may also not notice small culverts
or places where the water seeps under the road to reappear as a spring further down.
ASSESSING POTENTIAL BARRIERS
Once suspect stream corridors are found, an assessment must be made as to the ability of
fish to navigate the obstacle or reach.
There area many known barriers to fish passage in scientific literature. These include
aggressive predation through loss of cover and point source pollution. We will ignore
these issues in this paper.
We will be looking for physical barriers having to do with water flow only. These
barriers include; low flow reaches where the stream is too shallow, culverts and dams.
Shallow water
The Washington state Department of Fish and Wildlife has recommendations
(http://www.wdfw.wa.gov/hab/engineer/cm/culvert_manual_final.pdf) of minimum water
depths that fish can traverse. The depths vary with species, but we shall use the deepest
requirement to be conservative. They use 1 foot as a requirement for some salmon
species. This is a very conservative estimate and certainly most native fish can handle
this shallow water for quite a distance (large sturgeon excepted). If the person in the field
feels that the reach is below this depth for a significant distance and has the potential to
be a barrier it should be noted and reviewed.
Note: there may actually be sufficient
water here for passage but it is spread
over these rocks.
That was the case here before restoration.
Culverts
Culverts are the main impediment to salmon that is man made. They come in many
shapes. Once again, the WDFW
(http://www.wdfw.wa.gov/hab/engineer/cm/culvert_manual_final.pdf) has a
mathematical formula for ascertaining the ability of different salmon species to traverse
culverts. The Canadian government in B.C. has the following recommendations
(http://www.th.gov.bc.ca/publications/eng_publications/environment/references/Culverts
_and_Fish_Passage.pdf);
Grade
 Not to exceed .5% for culverts more than 24 meters in length unless baffles are
included.
 Not to exceed .5% for more culverts less than 24 meters in length unless baffles
are included.
 Not to exceed 5% at any time
Length
 Culverts under 24 meters in length can have water velocities up to 1.2 m/sec
 Culverts over 24 meters in length can have water velocities up to .9 m/sec
Water depth should be at natural levels or at least .23 meters at all times.
Drops at outlets should not exceed 1 foot, approximately 1 foot.
These recommendations were for new installations. In the field, pictures should be taken
of all suspect conditions for further analysis.
The main obstacle to fish passage with regard to culverts is the incident of a vertical drop
from the lip of the downstream end of the culvert which fish must jump.
Note the use of natural substrate for
bottom of culvert.
Plunges like this restrict passage.
DAMS
There may be buried, abandoned or obscured small man-man made dams along a stretch
of stream. According to the British Columbian Ministry of Transportation
(http://www.th.gov.bc.ca/publications/eng_publications/environment/references/Culverts
_and_Fish_Passage.pdf), all spills should be no more than .31 meters, or about 1 foot.
Here is an abandoned dam that stops fish
passage.
CONCLUSION
Finding and assessing fish barriers is a skill that will take plenty of field hours to master.
The main goal is survey as much of the stream as possible and to take note of any suspect
sections for further review.