Influence of Riparian Conditions on Physical Instream Habitats in Trout Streams in Southeastern Minnesota, USA

Lotic systems in agricultural settings can be impacted negatively by surface runoff and nutrients [52]. However, vegetated riparian zones can insulate lotic systems from land-use effects [53]. Wide intact continuous forest and grass buffers can intercept eroded soils before they enter streams, reducing instream fines and nutrient concentrations [54]. By comparison, forested buffers can provide woody structures important for physical instream habitat [55] and organic carbon inputs for food webs [13], while also reducing water temperatures and filtering out nutrients [56]. Grass and forb buffers are known to perform better in filtering than forested buffers [57], increasing suitable habitat for aquatic organisms [30], reducing overland flow of nutrients and increasing infiltration [54], and increasing stream velocity that flushes out fines and reduces embeddedness of coarse substrates [27,53]. Based on our findings, we suggest that certain physical conditions in the riparian zone can either protect suitable physical instream habitat from deteriorating or may help to restore previously damaged habitat. We observed mostly intact (continuity of buffer was not a measure, but visually noted) riparian buffers throughout the study area, with many being very wide (>90 m).

Within the study area, there were well-vegetated grassy buffers, a few restored prairies, and some forested areas, likely due to good land stewardship and compliance with the state mandated buffer law (Minnesota Statutes 2014, section 103B.101, subdivision 12, as amended in 2016 and 2020; https://www.revisor.mn.gov/statutes/cite/103F.48, accessed on 30 December 2023). Studies suggest that there may be a significant time lag between the establishment of new buffers and observable improvement in instream habitats; although, it may be possible to observe some improvements soon after buffer establishment [58]. We observed several “good” characteristics typically associated with ideal riparian zones, including long overhanging vegetation [31], minimal bare soil [27], bank reinforcement with natural rocks, and scattered shrubs and trees on and/or along the banks [12]. These features were correlated with better instream cover, more coarse substrates, reduced embeddedness, more pool habitat, and reduced fine sediments. Each of these “good” instream features likely can be correlated to wide, grassy expanses of the riparian zone across the study area [12,27,58]. We understand that seasonality can be a limitation, as physical changes can be easily tracked with varying seasons; however, due to logistics (short season, flashiness of streams, limited personnel), we were limited to mostly summer months.

Lotic systems have been subject to human activities for centuries and will continue to be subject to negative changes caused by the alterations of their floodplains, riparian ecosystems, and instream structure. Attempts have been made to enhance or restore degraded river ecosystems [59] with expectations to return stream functions to a desirable condition (i.e., better water quality, better physical habitat) [60,61]. However, results may vary, with both positive- and no-effect outcomes being possible [59]. We documented the physical characteristics that were assessed as negative or negatively influencing certain physical instream habitat features. Overall, undesirable riparian characteristics included narrow riparian buffers with mostly bare soil (i.e., lack of vegetation cover). Modeling determined that these characteristics correlated significantly to instream impairments such as excessive fine sediments, lack of proper riffle structure, lack of coarse substrates, and high width-to-depth ratios. Although most study sites were buffered, we did observe riparian areas that were minimally in compliance with the buffer law (i.e., mean width of 15 m, minimum width of 9 m). Narrow riparian zones are less effective in protecting streams and capturing eroding soils [62]; this likely explains our observations across many study sites and we also noted grazing livestock across many sites.

Unprotected riparian zones left open for livestock to freely graze also have negative impacts on streams due to reduced vegetation volume [55], failing (collapsing and eroding) banks, and stream widening, allowing sediments to enter the water column [45]. On many occasions, we halted field measurements due to the presence of livestock grazing the banks and wading in the water to drink. Such livestock impacts can be reduced by fencing that would limit livestock stream access only to designated watering locations, while also protecting and enhancing stream processes [28]. In areas where livestock were grazing on and near banks, we often observed active channel widening (e.g., unstable collapsing banks). Wide streams often have higher width-to-depth ratios, which can create slower flows and reduced sediment transport, effectively embedding coarse substrates and altering physical habitat structure [44]. The lack of riffle structure likely can be explained by the lag time from the implementation of improved riparian conditions and the instream response to that improvement [63], with the desired conditions just not yet observable. We likely will continue to see physical instream habitat improvements over the next several years, with maximum effect potentially requiring one or more decades [63,64].

The Whitewater River catchment has a long history of undesirable forest management and agricultural practices beginning in 1853, the year farming practices began in the region [42]. Despite determining that the “healthiest” part of the catchment was the Middle Fork, fines dominated physical instream habitat throughout the Whitewater watershed. Due to its lengthy history of human alterations, the catchment, like many others in the region, suffers from the effects of generational trauma known as legacy sediments. In river science, legacy has been used in connection with diverse past events including natural processes (i.e., wildfires, floods) [1]; here, we use the term legacy sediments as it has been associated with human activities [65]. Land altering activities in the Whitewater catchment continue to negatively impact water quality, suffocate coarse substrates, and alter other physical habitat features. On land, the impact of land alterations is shown through meters thick accumulations of legacy sediments [27] that are then transported via floods during intense storm events [66]. The effects of land alterations are well known throughout the world (Sweden, [67]; Australia, [68]; Europe, [69]; U.S., [70]). To remediate such adverse impacts, revegetation, bank stabilization techniques, and protection of riparian corridors through fencing and livestock exclusion are all common river management activities [71]. River scientists often intervene to remediate negative impacts, but these efforts are not always successful due to insufficient river management knowledge and or appropriate target scale [40].

Over the last several decades, much has been learned about riparian zones, their importance, and their relationship with lotic systems. For example, riparian zones have the capacity to insulate streams from agricultural activities (i.e., capture soil runoff, retain nutrients) [27], provide subsidies for biological communities [55], cool-down stream temperature [72], support primary production [73], provide ideal habitats [74], provide cover [46], and support local communities (e.g., subsistence (fish), drinking water) [1]. Following decades of interdisciplinary research, scale and riparian zone width were identified as major factors in a healthy and functioning riparian ecosystem [71]. Rivers are arrayed in a hierarchical nature (i.e., different levels of organization with a top-down structure; upper levels affecting lower levels), which makes it difficult to manage rivers at different spatial scales [40]. Managing rivers at the proper scale can prevent undesired effects at lower organizational levels [40]. When managing riparian zones, acknowledging that their width and physical structure (e.g., grass and forest buffers) varies depending on river type, will aid in proper management approaches [7]; not all rivers and streams require the same buffer widths [75]. In the U.S., a minimum buffer width of 30 m is mandated around perennial rivers by the United States Department of Agriculture (USDA) and the National Forest Management Act (NFMA) [7] and increased from the prior 7.5 m minimum [75] to better capture nutrients and eroding soils. In New Zealand, a recent study [76] reviewed riparian management progress in perennial, low-order streams (second to fourth order) [28] and found that most streams had a variable buffer width of 2–5 m as a compromise with private landowners to maintain productive arable land near streams. A study in Australia [77] described different widths protecting against certain processes. For lowland floodplain perennial rivers, the minimum requirements are 28 m to moderate stream temperature and 29 m to improve water quality. However, for low-order, high-gradient streams, a 28 m minimum buffer width is required for temperature regulation and 38 m to improve water quality.

The importance of riparian zones to aquatic ecosystems is well studied [8,14,27,78,79], and the need to manage and protect these sensitive ecosystems is emphasized. In the mid-1990s, the need for guidelines to protect riparian zones (at a basic level) was initiated by several nations, including the United States, Australia, New Zealand, Canada, United Kingdom, Sweden, and South Africa [80,81]. To that end, strategies to protect riparian areas began initially with forested buffers (improved forest management) [75], later expanding to include grassy buffers [31,82]. Despite a deeper understanding (i.e., how to protect, issues with scale, determining necessary width, and so on), monitoring of important river features is often overlooked. Despite the relatively low cost of monitoring compared with ecosystem services’ (i.e., nutrient cycling, provisioning, cultural) benefits from environmental protection, monitoring often is neglected as it requires funding and long-term efforts by qualified researchers [83]. Protection of the riparian corridor must continue, which means allocating funding for protection, maintaining intact corridors with sufficient buffer widths relative to the landscape template [14], establishing riparian areas that are spatially heterogeneous in nature (combinations of native grasses, mixed forests, prairie meadows) [84,85] and installing exclusions for livestock [28,76].

To better manage riparian ecosystems, we need to acknowledge that there are basic (at times minimal) management standards that should be followed. Researchers proposed a framework of five activities to conserve and protect riparian corridors: education, inventory, protection, sustainable management, and restoration [86]. Still, management of riparian corridors is met with challenges and patterns to overcome. One trend regularly emerging is the myriad of national, regional, and local legislation affecting riparian corridors [87] often focused only on water quality while failing to include non-aquatic features of lotic systems (e.g., the European Water Framework Directive, and in the USA the Clean Water Act, National Environmental Protection Act, and Farm Bill). An exception to this trend was the U.S. National Wild and Scenic Rivers Act (WSR Act, 1968). The WSR Act has protected well over 20,000 km of streams and rivers across the U.S. that exhibit natural, cultural, or aesthetic qualities, to preserve water quality and other non-aquatic components vital to conservation. Trends in changing climate have demonstrated the magnitude of the effects of floods and droughts which alter the structure and functioning of riparian ecosystems, affecting the response and recovery trajectories of aquatic organisms to disturbances [88]. Climate trends coupled with human-influenced impacts have a compounding effect which makes the diagnosis of riparian conditions difficult due to lag-time of the onset of impacts and how rivers respond [89]. This is important to identify early on to better protect and conserve riparian ecosystems which can be heeded with regular monitoring. Although many of the tools needed to protect riparian and river ecosystems are utilized, they are not broadly applied together with the applicable standards needed to buffer lotic systems.