DOI: 10.5061/dryad.83bk3j9t1
title: Size-dependent effects of dams on river ecosystems and implications for dam removal outcomes

description: Understanding the relationship between a dam’s size and its ecological
 effects is important for prioritization of river restoration efforts based
 on dam removal. Although much is known about the effects of large storage
 dams, this information may not be applicable to small dams, which
 represent the vast majority of dams being considered for removal. To
 better understand how dam effects vary with size, we conducted a
 multidisciplinary study of the downstream effect of dams on a range of
 ecological characteristics including geomorphology, water chemistry,
 periphyton, riparian vegetation, benthic macroinvertebrates and fish. We
 related dam size variables to the downstream-upstream fractional
 difference in measured ecological characteristics for 16 dams in the
 mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic
 residence times (HRT) ranging from 30 minutes to 1.5 years. For a range of
 physical attributes, larger dams had larger effects. For example, the
 water surface width below dams was greater below large dams. In contrast,
 there was no effect of dam size on sediment grain size (though the
 fraction of fine-grained bed material was lower below dams independently
 of dam size). Larger dams tended to reduce water quality more, with
 decreased downstream DO and increased temperature. Larger dams decreased
 inorganic nutrients (N, P, Silicate), but increased particulate nutrients
 (N, P) in downstream reaches. Aquatic organisms tended to have greater
 dissimilarity in species composition below larger dams (for fish and
 periphyton), lower taxonomic diversity (for macroinvertebrates), and
 greater pollution tolerance (for periphyton and macroinvertebrates).
 Plants responded differently below large and small dams, with fewer
 invasive species below large dams, but more below small dams. Overall,
 these results demonstrate that larger dams have much greater impact on the
 ecosystem components we measured, and hence their removal has the greatest
 potential for restoring river ecosystems.
description: See methods described in main manuscript submitted for
 publication.
description: # Size-dependent effects of dams on river ecosystems, and implications for
 dam removal outcomes ##
 Description of the data and file structure File list (files found within
 DataS1.zip) brown_et_al_2024_data.xlsx brown_et_al_2024_impound_wc.csv
 brown_et_al_2024_indep_var_values.csv
 brown_et_al_2024_spearman_indep_vars.csv
 brown_et_al_2024_depend_var_descriptions.csv
 brown_et_al_2024_depend_var_up-down_values_and_fract_diffs.csv
 brown_et_al_2024_bray_curtis_similarities.csv File descriptions
 brown_et_al_2024_data.xlsx - Supporting data for Brown et al. 2024. Data
 files in the tabs in the Excel file are also provided as .csv files and
 are described below. Blank cells in all data tabs indicate missing values.
 Brown_et_al_2024_impound_wc.csv - Impoundment water chemistry profiles and
 stratification information. Data for the Gunpowder River (GUNP) was
 obtained from the City of Baltimore, which owns and monitors Prettyboy
 Reservoir; site GUN0339 is at the gatehouse, affixed to the dam; site
 GUN0401 is 305 m upstream. Each impoundment is identified by its site code
 (Table 2, Brown et al 2024). DO (mg/L) for HOPE and SUNR were not recorded
 when measurements were taken; they were calculated from DO% and Temp (C),
 but not corrected for depth. DO% for GUNP sites was not corrected for
 depth. brown_et_al_2024_indep_var_values.csv - Descriptions and values of
 independent variables. Blank cells indicate missing values.
 brown_et_al_2024_spearman_indep_vars.csv - Spearman correlations of
 independent variables related to impoundment or watershed characteristics.
 Spearman correlation coefficients are above with italicized P values below
 them; significant relationships are in bold. “Diff.” refers to downstream
 minus upstream differences. All variables were log-transformed prior to
 being correlated (though this does not affect results with Spearman
 correlations). brown_et_al_2024_depend_var_descriptions.csv - Description
 of dependent variables.
 brown_et_al_2024_depend_var_up-down_values_and_fract_diffs.csv - Values of
 dependent variables used for analysis and fractional differences. See
 Brown_et_al_2024_depend_var_descriptions.csv for description of variables.
 Downstream-minus-upstream fractional differences were calculated as
 2*(downstream value – upstream value)/(downstream + upstream value),
 except for downstream-upstream species compositional similarity
 coefficients, which we calculated for each taxonomic group (periphyton,
 macroinvertebrates, fish, and plants) using Bray-Curtis similarity.
 Difference values for dependent variables used to calculate the multiple
 regression values shown in Table 3 in Brown et al 2024. Fish sampling was
 not done at all sites because of budget limitation, inability to
 effectively sample some sites and occurrence of other dams in the
 drainage. Blank cells indicate missing values.
 brown_et_al_2024_bray_curtis_similarities.csv - Bray Curtis percent
 similarities for periphyton, macroinvertebrates, fish, and riparian
 plants. FP - Flood plain; UP = Upland. Blank cells indicate missing
 values. The files in this dataset include descriptions and values of
 measured variables used in analyses, chemistry and stratification
 information on impoundments, and upstream-downstream Bray-Curtis
 Similarity values for biota studied. The readme.txt file describes
 individual data files. Data are in both the .xlsx file and in individual
 .csv files. Blank cells in the files indicate missing values. Data can be
 used to re-evaluate study results and for new studies on the influence of
 dams on rivers.