The collection of the material can be done once or several times during the dry period. The material needs to be collected before the flow resumption, and the time during which CPOM had accumulated (dry period duration at the collection date) estimated (with a 1-2 week precision, using either loggers, repeated observations, local knowledge, etc, depending on your situation).
At each collection date, estimate the area of the selected reach (length * average active channel width). Then, calculate the surface from which you need to collect CPOM to sample at least 5% of the reach surface using 1 m² quadrats. For example, for a stream with an active channel of 5 m, the reach will be 5*10= 50 m long and has a surface of 250 m2, indicating a minimum of 12 quadrats will be required.
Note in case of active channels <2m (eg. headwater streams), the size of the quadrat can be adapted (eg. using quadrats of 0.5*0.5, 0.2*0.2 m), respecting 1) the definition of the reach length, 2) the rule of collecting material from at least 5% of the reach surface area, and 3) making sure the heterogeneity of the reach is encompassed. If the reach is really small (1st order streams), ensure you can collect ~60g of leaves and indicate the associated sampled surface.
Then collect the 3 types of material from each quadrat, which you place across the active channel, with half of quadrats in the center of the channel and the other half on the margins of the active channel (this may not apply for rivers with active channels < 2 m). Overall, make sure the quadrats are being placed so you encompass the heterogeneity of the whole reach (for example, place transects equally spaced along the reach and spread the quadrats across them).
The 3 fractions are CPOM, biofilm/algal mats and riverbed sediments.
CPOM includes all organic materials deposited at the surface of the streambed, including leaves, wood, fruits, catkins and the “fresh” vegetation (ie. herbs, shrubs) developing. CPOM can be collected by hands and stored in plastic bags. Each CPOM type has to be stored separately, but pooling together all samples (eg. central and lateral samples) per reach (because the interest is not on within-reach heterogeneity). For large pieces of wood in jams originating from flowing periods, estimate roughly its volume.
Biofilm/algal mats are the layers deposited over the sediments which can remain from the flowing period. For each quadrat, once the CPOM has been collected, subsample an area of 20*20 cm to collect biofilm/algal mats by removing mats and/or scrapping stones with a razor blade into a different plastic bag (Ziploc), tightly closed with the air-expelled. Pool together all samples per reach. If the riverbed has no biofilm/algal mats, skip this step.
Riverbed sediments are the sediments composing the riverbed. Once the biofilm/algal mats have been collected, collect some riverbed sediments from a depth of 0-10 cm from the quadrat using a spoon or a shovel (no surface area needed, try to get ~3 L in total across the reach). Collect the fine fraction only (typically gravel, sand and clay). Sometimes, you may need to remove large cobbles to access the finer fraction underneath; however, in any case don’t go deeper than 10 cm). Store this sediment into a different plastic bag (Ziploc), tightly closed with the air-expelled. Pool together all samples per reach. If the riverbed is composed of coarse sediments or bedrock, skip this step. A 2 mm sieve can be helpful to select the fine fraction directly in the field.