Have our rivers got better or worse in the last 100 years?

Another question I was asked in my most recent interview, on RadioLIVE, was, given the more 100-odd years of abuse suffered by many of our rivers, have our rivers on the whole recovered from their historically degraded states, or not?

As always, the answer is not a straight-forward one. In part because, 100 or 150 years ago, we did not have freshwater scientists measuring the health of our rivers using metrics that could be compared over time, as we do now. The sort of measurements we make now have only been made in a systematic way for the last few decades.

As I explained in the interview, the source of pollution in the past was mainly what we call point-source discharges, effluent piped into rivers and streams from factories, sewage treatment plants and the like. By the mid-1990s, we had largely brought these discharges under control - they still occur, but they are monitored closely and various measures have been taken to reduce the level of effluent from each source as much as possible.

Taking over from these known sources are what we call diffuse discharges - often referred to as agricultural run-off - water containing animal effluent and fertiliser, rich in nutrients. These are much less obvious than point-source discharges, but no less damaging. In fact because they have become so pervasive, and because they can cause changes in the chemical balance of waterways, they may pose a greater threat overall than point source discharges ever did. Coupled with this is the exponential increase in extraction from our rivers for irrigation, which also depletes the water carried by waterways (thereby also reducing its ability to dilute pollutants).

So, I think my short answer to this question is that river pollution historically (eg, 100 years ago) was characterised by pollution 'hot-spots' - stretches on rivers and streams where pollution was severe - sometimes even leading to fish dying en-masse. But many rivers (and regions) probably remained relatively pristine - I suspect that Canterbury was one of those regions. Today, we rarely have the severe localised pollution leading to the horror of fish-kills, but pollution (or 'poor water quality', to use the preferred term in policy circles) is much more pervasive.

Which of these is worse? To know for sure, we would probably need a freshwater scientist in a time machine. But given our much improved knowledge or water quality, ecosystem health and the like today, we don't have the excuse of ignorance that our ancestors may have offered for their abuse or neglect or rivers 100 years ago.

Photo top: Today fish kills are rare but not unheard of. Fish (all pilchards) were found dead in their thousands on the lower Matai River in Nelson in 2013. Photo: MARTIN de RUYTER/Fairfax NZ