Assessing the effects of EXtreme summer flooding on STREAM ecosystem successional processes (EXSTREAM).

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A major variable influencing river ecosystems is disturbance, particularly that due to flooding. Floods maintain ecosystem diversity and redistribute energy, nutrients, sediment and biota in rivers. The biota of rivers (e.g. invertebrates and fish) may survive floods either by withstanding the disturbance (resistance) or recovering rapidly (resilience). Although disturbance to river ecosystems due to flooding has been relatively well studied, many studies have a restricted pre-disturbance series of data against which to evaluate flood effects and rarely in terms of their successional development and few have compared different flooding regimes. This in part reflects minimal continuous, year-on-year monitoring of stream communities against which to measure any disturbance effects.In Glacier Bay National Park in southeast Alaska, we hold long-term continuous data sets (up to 35 years for some streams) detailing stream community evolution following glacial retreat. These data have provided unique insights into primary succession (change over time) of stream invertebrates and fish. For one stream (Wolf Point Creek; WPC) a highly detailed trajectory of community change has been assembled since 1977, showing how the stream has evolved from one dominated by a few species of Chironomidae (non-biting midges) to one more recently containing a diverse community of invertebrates (insects/non-insects) and juvenile salmonids. The summer of 2014 has seen record-breaking prolonged high rainfall in Glacier Bay and southeast Alaska generally, with the wettest June on record (189mm = 231% higher than normal). Furthermore, in July precipitation was 180% greater than normal with 211mm, making it the second wettest July on record. Over 50 mm rain fell in Glacier Bay on July 12 and > 90 mm in a 4-day period. Heavy precipitation has continued into August with the 12th being the wettest August day on record. This has created a series of large, recurrent and atypical flood events during the summer (some events 8x median flow), the effects of which we propose to investigate in detail. Three major avenues of research will be followed: (1) the effect of the flooding on the stream habitat and morphology across streams of different ages, (2) an investigation of the effect of the flooding on the primary successional framework (invertebrates, fish) in Wolf Point Creek, and (3) an investigation into the effects of the flooding on streams of different ages to provide information on how streams at different stages of successional development respond to flooding.We will compare the results with those obtained from one massive flood disturbance in November 2005 in the same system but driven by continuous heavy rainfall (over 650mm in less than 72 hours). The size of this previous event is evident when compared against an average annual rainfall of 920mm. This research will provide a unique study in comparison of different flooding regimes at different times of the year with respect to geomorphological and biotic responses. These extreme flooding events are expected to become more common in the future with climate change, thus our findings will inform mitigation strategies to manage the effects of flooding on ecosystem integrity.