This started out as an e-mail discussion with a friend. A graph (below) sent to me showing that over the past years, average flow appears to be consistently greater than the historical median flow for the past 58 years. An interesting observation that was worth looking into further.
At its heart though the argument is flawed. This is because 'average' flow will nearly always be higher than 'median' flow. So its an apples to oranges comparison. However that doesn't necessarily mean its not true, just that the proof being supplied isn't correct to support the assertion. So I thought I'd look into it further and see if I could find the data to either prove or disprove the theory, as I was rather curious myself at this point.
Several things need to be taken into account when deciphering this puzzle. Historical flow broken down by year was something that needed to be looked at. An average over 58 years is one thing but a closer look at the actual yearly flows might lend some additional information. Below is a chart showing the yearly flow for the White R. at Nora, our test section for the rest of this exercise.
The orange highlighted sections are the 10 years with the highest average flow for the White River. Unfortunately, this doesn't help too much as the high flows are pretty well spread out over the entire graph with no overly heavy trend toward worse flows at the present time. So maybe if we look at precipitation on a yearly basis.
So here we have the yearly rainfall for Indianapolis for the past 50 years. Indianapolis averages right around 40 inches of precipitation per year. No real obvious trends here, some years more than others and some less. So lets overlay the 2 previous graphs and see if there are any correlations.
So here we have average yearly streamflow in cubic feet per second (cfs) on the left hand of the graph and in purple. On the right (and in blue) you have the total yearly precipitation (in inches) for those same years. This turns out to be a pretty cool graph with good correlation, basically revealing that the more rain we get in a year, the higher the average flow of the White R. for that year. This isn't any big surprise though as you would expect the result, and it certainly doesn't answer the question of if the original observation of higher flow is true or not.
The problem is that every year is different. Different precipitation amounts, different weather patterns, different environmental conditions. It is hard to look at a 1 or two year time period and make a judgement call, even when compared to a historical or long-term average. What we need to do is think in bigger terms, longer time periods.
So this next graph takes the 5 year moving average of flows for the White R. This basically groups several years together and tends to average out short term variability. On the graph above, it appears via the trendline to be increasing, but even 5 years isn't a great indicator. Development takes a long time, so while this would accont for some of the weather trends, it doesn't give you an accurate enough view. So lets zoom out:
To 10 years. Trendline is still there, slightly more positive but the graph as a whole doesn't look too convincing. This look smooths out some of the ups and downs you get with 5 years, but you still see some height at the start of the chart that is similar to averages toward the end. And 10 years might reflect a lot of environmental balance, as well as local development, but it still doesn't capture the changes of an entire city. So we zoom out some more...
To a 20 year moving average. Now we finally start to get the picture. This is a large enough time block to wash out random and short term weather effects. This is also a long enough block of time to smooth out other short term environmental factors. It also now starts to encompass multiple development sites over the city encompassing bigger projects and larger reference scales.
Now we actually see an even stronger correlation and a more obvious and apparent increase in flow for the White R. that can be readily picked up visually on the graph. As it turns out, it seems that the average flow for this stretch of the White R. has increased something to the tune of 20 percent or more over the past 75 years. This timeline is long enough to cancel out local or short term weather anomalies. So how does this flow increase occur?
It occurs due to basic changes in the city itself. From increased development which leads to more impervious surfaces, thereby increasing runoff. Changes in location of development, often times into the river floodplain itself , a result of less remaining space. Some of the only areas left to develop, short of tearing down old buildings, is actually in areas that would previously go undeveloped (in the flood plain areas) due to increased risk of loss. Of course, this more modern development tends to also levee or protect itself more than in the past, changing the basic limits or "out-of-bounds" that the river can occupy for a given volume of incoming rain and drainage. As such, even normal rainfall amounts now end up in the river either faster than before or, once in the river, tend to stay more restricted inside the rivers core path. This is why average flows on the White R. are definitely increasing, slowly but surely.
The question is, what effect, if any this will have on the fishery as well as the other wildlife that calls the river home?