Aqua Incognita: the unknown headwaters

Abstract

Running water comprises just over one millionth of the world's water. The importance of those streams and rivers as a resource for human welfare and biodiversity, however, is far out of proportion to that minuscule fraction. This explains why protecting running waters (the flow regimes, water quality and biota) is such a vital concern for society. Yet for all the focus and concern, how much do we actually know about these running waters, and the lotic habitat they comprise?Consider what would happen if one asked any national environmental authority to assess the basic chemical and ecological status of running waters. At the river mouths, there would be enough information to make a reasonable assessment of the status. But somewhere on the way upstream, available data would run dry, long before most stream channels did (in non-arid regions).In Sweden, with an ambitious programme for monitoring and assessing surface waters, it came as a surprise several years ago to realize that the length of all perennial streams on the country's maps was not known. When that was modelled in the form of a 'virtual network' from a 50 m × 50 m digital elevation model, the total length turned out to be 530 000 km (ca 1 km/km 2 ), which was double the previous estimates. The length was independently confirmed by another group using remote sensing data (Esseen et al., 2004). Further analysis of the virtual network revealed that over 90% of the stream length had catchment areas under 15 km 2 . Although this might seem merely of academic interest, 15 km 2 is the lower limit for what has been surveyed on a national scale in Sweden. Does this mean that we have missed something important in our assessment of water resources?When the chemistry of all flowing headwaters in a single 78 km 2 catchment was compared to the 2000 Swedish national survey of running waters, there was as much variability within the headwaters of that forested catchment as could be found in a statistically representative sample of over 260 000 km 2 of Sweden's boreal forest waters (Temnerud and Bishop, 2005). Other studies on biota have not just found such headwaters to be teeming with biodiversity, but also found species that are endemic to headwaters (Meyer et al., 2007). Discrete inquiries were made to see if national agencies in other countries of North America and Europe had come further in the documentation and assessment of headwaters. The answer was 'no'. There are, however, some significant efforts (e. g. Hutchins et al., 1999; Smart et al., (2006). But since many first and second order streams are not on the US maps, and the assessment went up to fifth order streams, headwaters are likely to be seriously underrepresented even in that landmark survey. 2001; Likens and Buso, 2006). The most notable is the US Environmental Protection Agency's (EPA's) 'Wadeable Stream AssessmentIn most regions, the overwhelming majority of stream length lies beyond the frontiers of any systematic documentation and would have to be represented as a blank space on the assessment m...

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