The seasonality of river flooding in North Britain displays considerable spatial variation. This paper identifies the geographical patterns of flood seasonality, using a database of events exceeding modest flood-flow thresholds at each of 156 gauging stations, and seeks to explain them in terms of climatological and catchment characteristics. Floods are found to occur at all times of year, but most rivers register at least 78% of events in the October-March half-year, and these generally occur later in the year with distance from west to east. However, notable exceptions are superimposed upon this general pattern and, in particular, two areas of less pronounced seasonality occur on north-facing parts of the east coast. Seasonality is characterized using three complementary methods, including a four-fold seasonal classification which summarises the patterns found. In order to explain these patterns, reference is made to the seasonality of storm rainfall, soil moisture deficits, catchment size and lake storage. Seasonality class is correctly explained by reference to these catchment characteristics in 74% of cases using discriminant analysis. The work is presented as an advance in the understanding of flood generation and, ultimately, in the assessment of flood risk.