Hello Necla, here's a research that considered this particular relation. Check out Figure 4. where we separated "Well, medium and unprotected" watersheds (according to their riparian forest cover).

As with any other relation with forests and water, universally valid declarations are difficult to elaborate. 

Here's other study about riparian protection in bio-indicators (macroinvertebrates) however it is only in Spanish. 

I would finally like to comment that point source pollution, for example a specific discharge of contaminated water, can make it difficult to determine the role of riparian forests. 

Article Human pressure on water quality and water yield in the upper...

One of the best ways to protect, conserve water quality is to consider a riparian buffer along perennial and intermittent streams.  Forest and vegetated riparian areas help to proviide stream shade, roots to hold stream banks, limit pollutants from adjacent areas, etc.  There may be differences in opinion in how much of the riparian area needs protection, or varying degrees of protection and management, but some of these questions are best applied to specific circumstances, as not all riparian areas look the same, and can vary from perhaps 50%.  Riparian areas effects on streams are most noticed when riparian areas are abused or removed.  It is probably not unusual for riparian areas to also support many habitats and species.  My answer is yes, there are connections between riparian areas and water quality, but they are complex and variable, the degrees dependent on the circumstances.  I am not in my computer, but probably have a few things to send, but there should be volumes on the internet

Perhaps these will help in your considerations.

Apparently ran out of space with the above, so a few more of my files. 

There's much evidence that riparian buffers need to be 30-50 m wide to protect water quality for lotic zoobenthos:

Vadas, R.L. Jr. 1997. Assemblage structure of riparian and drifting invertebrates along environmental gradients in two streams of southern British Columbia. DOE FRAP (Canada Department of the Environment, Fraser River Action Plan) 98(28): 62 pp. (http://research.rem.sfu.ca/frap/9828.pdf).]

Vadas, R.L. Jr. 1998. Human impact on aquatic and riparian ecosystems in two streams of the Thompson River drainage, British Columbia. Pages 13-30 in M.K. Brewin and D.M.A. Monita (eds.). Forest-fish conference: land management practices affecting aquatic ecosystems. Natural Resources Canada, Forest Service,

Northern Forestry Center Information Report NOR-X-356. Edmonton, AB (http://cfs.nrcan.gc.ca/pubwarehouse/pdfs/11639.pdf).

-Bob Vadas, Jr.

From a conceptual standpoint, TSS typically increases if riparian-buffer strips (RBSs) are too narrow. That can also exacerbate solar radiation, and such summer heating can cause aquatic-weed blooms to heighten diurnal fluctuations in DO and pH. Narrower RBSs also cause greater seasonal fluctuations in water temperature, so less physicochemical (including instream-flow) modulation is a big concern.

Please read the paper by Lowrance et al. (Table 5 and 6).

Lowrance, R., Altier, L. S., Newbold, J. D., Schnabel, R. R.,

Groffman, P. M., Denver, J. M., Correll D.L., et al. (1995).

Water quality functions of riparian forest buffer systems in

the Chesapeake Bay Watershed. U.S. Environmental

Protection Agency, Washington.

Here’s another good report (online) of riparian relevance for lotic water quality and biota:

Platts, W.S. 1990. Managing fisheries and wildlife on rangelands grazed by livestock: a guidance and reference document for biologists. Nevada Department of Wildlife. Reno, NV. Vp. (https://archive.org/details/managingfisherie13plat).

-Bob

Thank you so much dear Karim Musalem

Dear William F. Hansen thanks for your contribution

Dear Robert L. Vadas thank you so much for your contribution

Thanks dear Jolanta Dąbrowska

Dear Necla ,did you find your answer? Is there any confirmed relationship between riparian width and water quality parameters? I couldn't find any but descriptive studies.

Further FYI for the need for 30-m buffers:

Belt, G.H., and J. O’Laughlin. 1994. Buffer strip design for protecting water quality and fish habitat. Western Journal of Applied Forestry 9(2): 41-45 (cf. https://www.osti.gov/scitech/biblio/7201650).

Dosskey, M.G., M.J. Helmers, and D.E. Eisenhauer. 2008. A design aid for determining width of filter strips. Journal of Soil and Water Conservation 63: 232–241 (https://www.fs.usda.gov/treesearch/pubs/31091).

Erman, D.C., and D. Mahoney. 1983. Recovery after logging in streams with and without buffer strips in northern California. University of California, Water Resources Center Contribution 186: 50 pp. (http://vwvw.krisweb.com/biblio/ncc_ucdwrc_ermanetal_1983_mahoney.pdf).

Macdonald, J.S., ed. 1994. Proceedings of the Takla Fishery/Forestry Workshop: a review. April 1, 1993, Prince George, B.C. Canadian Technical Report of Fisheries and Aquatic Sciences 2007: 104 pp.

Mohamedali, T. 2014. A potential approach for developing prescriptive buffer widths for temperature TMDLs. Washington Department of Ecology, Environmental Assessment Program Technical Memorandum 14(050): 19 pp.

Quinn, T., G.F. Wilhere, and K.L. Krueger, eds. 2020. Riparian ecosystems, volume 1: science synthesis and management implications (updated version). Washington Department of Fish and Wildlife, Habitat Program. Olympia, WA. 304 pp. (https://wdfw.wa.gov/publications/01987).

Sweeney, B. W., and seven coauthors. 2004. Riparian deforestation, stream narrowing, and loss of stream ecosystem services. Proceedings of the National Academy of Sciences of the United States of America 101: 14132-14137 (http://www.pnas.org/content/101/39/14132.full).

Sweeney, B.W., and J.D. Newbold. 2014. Streamside forest buffer width needed to protect stream water quality, habitat, and organisms: a literature review. Journal of the American Water Resources Association 50: 560-584 (http://onlinelibrary.wiley.com/doi/10.1111/jawr.12203).

-Bob

good question