Table 1. Number of observations per category of the categorical variables (including missing values), and the quantiles (5, 50, 95%), mean and percentage of missing values of the continuous variables.

Substrate (number of observations)
Silt, sand and gravel	Sand	Gravel and boulder	Gravel	Silt and sand	Sand and gravel	Rock and gravel	Missing
662	339	279	255	219	54	34	54
Continuous variables
Quantiles	Alkalinity	Velocity	Nitrate	Total phosphorus	Width	Depth
	(meq CaCO3 L-1)	(m s -1 )	(mg L -1 )	(μg L -1 )	(m)	(m)
5%	0.4	0.10	0.11	19	2	0.1
50%	2.6	0.50	2.51	110	8	0.4
95%	6.6	0.80	17.00	796	82	1.5
Mean	2.9	0.48	4.25	256	18	0.5
Missing (%)	52	60	17	35	14	28