The CDA dam safety guidelines include recommended methodologies for estimating probable maximum flood (PMF) events, particularly for rainfall-driven PMF events, that are widely accepted. For snowmelt events, CDA guidelines suggest assessing two potential spring PMFs: the PMF computed with the spring probable maximum precipitation (PMP) and snow accumulation with an annual exceedance probability (AEP) of 1/100, and the PMF computed with the 1/100 AEP precipitation on a probable maximum snow accumulation (PMSA). Both approaches are reasonable and generally well accepted by the industry.  Reservoirs are typically assessed to determine their critical duration, which can be especially important for mining dams that are either designed and operated to store a significant amount of flood runoff volume, or designed without a spillway (i.e., to store all of the flood runoff volume without discharging). Long duration floods are essential for assessing these types of reservoirs, as they often produce the most volume and can also produce the highest peak flows. While the CDA does not specify duration limits for the two suggested Spring PMF approaches, the PMP and 1/100 AEP precipitation are typically associated with a short-duration storm event, that could last hours to several days. While there are accepted, well-used methodologies for estimating short-duration rainfall depths, methodologies for estimating long-duration events are not well established within the industry. Methodologies for defining the temporal distribution of long-duration rainfall is also not well understood and is critical when estimating a Spring PMF that includes snowmelt. This paper presents a novel approach for estimating long duration snowmelt dominated Spring PMFs, that are consistent with the two Spring PMF approaches defined by CDA, as well as considerations for sensitivity scenarios, for estimating Spring PMFs at sites with long-duration events. The Spring PMFs were used to assess the performance of a mining dam in Interior British Columbia, where the review of recorded hydrological data clearly shows that extreme local flood conditions are caused by a 30 to 60 day spring freshet.