To cite RIVER in publications use:

Stephen B. Montgomery

  Xin Li, Yungil Kim, Emily K. Tsang, Joe R. Davis, Farhan N. Damani,
  Colby Chiang, Zachary Zappala, Benjamin J. Strober, Alexandra J.
  Scott, Andrea Ganna, Jason Merker, GTEx Consortium, Ira M. Hall,
  Alexis Battle, Stephen B. Montgomery (2016) The impact of rare
  variation on gene expression across tissues bioRxiv, doi:
  https://doi.org/10.1101/074443 URL:
  http://biorxiv.org/content/early/2016/09/09/074443

A BibTeX entry for LaTeX users is

  @Article{,
    title = {The impact of rare variation on gene expression across tissues},
    author = {Xin Li and Yungil Kim and Emily K. Tsang and Joe R. Davis and Farhan N. Damani and Colby Chiang and Zachary Zappala and Benjamin J. Strober and Alexandra J. Scott and Andrea Ganna and Jason Merker and GTEx Consortium and Ira M. Hall and Alexis Battle},
    year = {2016},
    url = {http://biorxiv.org/content/early/2016/09/09/074443},
    journal = {bioRxiv},
  }