Spatial and Temporal Variations in Phosphorus Loads in the Illinois River Basin, Illinois USA

Gregory F. McIsaac, Timothy O. Hodson, Momcilo Markus, Rabin Bhattarai, Daniel Chulgi Kim

Research output: Contribution to journalArticlepeer-review


Total phosphorus (TP) loads in many rivers in the north-central United States have increased, including the Illinois River at Valley City, Illinois, USA, which increased 39% from the periods 1989–1996 to 2015–2019 despite efforts to reduce loads from point and nonpoint sources. Here, we quantify long-term variations in phosphorus (P) loads in the Illinois River and its tributaries and identify factors that may be causing the variations. We calculated river loads of dissolved (DP) and particulate P (PP), total and volatile suspended solids (TSS and VSS), and other potentially related constituents at 41 locations. DP loads generally increased and PP and TSS loads generally decreased from 1989–1996 to 2015–2019. During 1989–1996, P accumulated in the lower basin between Marseilles and Valley City (excluding monitored tributaries). This portion of the basin is very flat and accumulates sediment. During 2015–2019, this section shifted from being a net sink to being a net source of P, accounting for 78% of the increased TP load at Valley City. We present evidence supporting several mechanisms that could have caused this shift: increased DP and chloride loads, reduced sulfate and nitrate concentrations influencing ionic strength and redox potential in the sediments, and increased VSS loads at Valley City possibly indicating greater algal production and contributing to hypoxia in lower river sediments. Additional research is needed to quantify the relative importance of these mechanisms.

Original languageEnglish (US)
Pages (from-to)523-538
Number of pages16
JournalJournal of the American Water Resources Association
Issue number3
StatePublished - Jun 2023


  • freshwater
  • nonpoint source
  • phosphorus
  • point source

ASJC Scopus subject areas

  • Ecology
  • Water Science and Technology
  • Earth-Surface Processes


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