Original model reference publication: Egholm DL, Knudsen MF, Clark CD, Lesemann, JE (2011) Modeling the flow of glaciers in steep terrains: The integrated second-order shallow ice approximation (isosia). J Geophys Res, http://dx.doi.org/10.1029/2010JF001900 The surface processes model (spm) is the core landscape evolution model, while the integrated second-order shallow ice approximation (iSOSIA) is a higher-order glacier evolution model which considers both the longitudinal and transverse stresses that govern ice flow over steep terrain in mountain glaciers. Models based on the shallow ice approximation (SIA) only consider longitudinal stresses, as the SIA was developed to calculate the flow of ice sheets over shallow terrains. iSOSIA limits sliding velocity with basal shear stress rather than allowing velocity to increase exponentially as in the SIA. Previously released versions of spm:spm-3.3.8 (Bernard et al., 2021, GRL): https://zenodo.org/records/4568845spm-3.4.3 (Scherler and Egholm, 2020, JGR-ES): https://github.com/davidlundbek/iSOSIA/releases/tag/Scherler&Egholm Full Changelog: https://github.com/annvrowan/isosia/commits/spm-3.4.8