Transient Organic Jams in Puerto Rican Mountain Streams After Hurricanes

Extreme storms in forested environments commonly increase inputs of coarse particulate organic matter (CPOM) and large wood (LW) to streams. Features that can trap CPOM and LW in mountain streams include protruding boulders and bedforms, mid-channel bars, and standing trees on forested islands and along the channel banks. These organic accumulations can become so large that they span the bankfull channel width and create a temporary dam and associated backwater pool habitat, or the accumulations can be predominantly along the channel margins. We refer to both types of accumulations as transient organic jams (TOJs). TOJs are of interest because of their potential geomorphic and ecological effects in channel and floodplain environments. We use data collected from first- and second-order streams of the Luquillo Mountains of northeastern Puerto Rico following two major hurricanes, Irma and Maria, which occurred in September 2017. We examine the location, characteristics (length, height, volume), and geomorphic effects of TOJs in channel segments representing a range of drainage areas and channel gradients. We ask the following three questions: (1) Is there a threshold - defined in terms of drainage or total stream power - within a river network that separates channel segments in which TOJs are predominantly channel-spanning from channel segments in which TOJs are predominantly smaller than the bankfull channel width? (2) Does the downstream spacing of TOJs correlate with variables such as drainage area, stream power, or channel gradient? (3) What variables best predict the volume of organic matter within individual TOJs or within a channel segment? Datasets include multiple TOJs along each of 12 stream segments. Data analysis with multiple linear regressions indicates that downstream spacing and volume of TOJs correlate significantly with bankfull channel width. Using the AICc model selection method, Strahler stream order has the most influence on the probability of TOJs being marginal or spanning.