The hydrometrical observations on the rivers in Bulgaria for many years have revealed, that the main reasons for the ice phenomena on the rivers are the low air temperatures in a sequence of many successive days, the low flow regime, the cold water inflow, the appearance of submerged ice, pulpy ice, floating ice, coming from the upper river reaches etc. Sometimes the freezing of the river reaches and melting the ice might repeat in a season.
     The regular observations on the ice phenomena in Bulgaria have started since 1936/37 on the Danube River, the reason being the navigation of this transeuropean river and because of the requirements of the International Danube River Commission. Right at the beginning they have been done on 15 places along the river sector, but later the number of measuring sites has been reduced to 7 (Novo Selo, Lom, Oriachovo, Somovit, Svishtov, Russe and Silistra).
     Regular observations for the ice regime on the inland rivers have started since 1950 on all hydrometrical gauge stations (Ivanov and all, 1961; Marinov, 1990).
     The analysis of the hydrological information, stored on the information data bank in the Division on River Sediments and Fluvial Processes, have revealed a great diversity in the time of appearance/obsolete and the duration of the ice regime (Table 1) on the rivers in Bulgaria.

Table 1.  The beginning and the end of the ice regime, its mean duration and frequency on the rivers in Bulgaria.

River	Hydrometrical Gauge Station (HyGSt)	Altitude of the HyGSt	Ice phenomena 1960/61-89/90 г.		Nср. л.явл. (days)	Pср. л.явл. (%)	Nср. залед. (days)	Pср. залед. (%)
			Begining	End
Lom	Traikovo	46,07	1,12	6,03	25	93	6	22
Ogosta	Kobiliak	74,71	1,12	4,03	19	70	14	17
Iskar	Orehovitza	31,20	7,12	28,02	16	80	9	13
Vit	Sadovetz	107,17	1,12	4,03	32	70	27	53
Ossam	Izgrev	40,32	20,11	28,02	19	45	21	5
Jantra	Karantzi	30,90	6,12	2,03	15	53	9	13
Rosica	Vodolej	67,06	11,11	14,03	19	87	12	57
Kamchia	Poda	0,00	7,12	16,02	14	27	12	20
Maritza	Plovdiv	155,08	16,12	9,02	8	33	0	0
Maritza	Svilengrad	46,88	7,12	16,02	8	50	6	7
Topolnitza	Koprivshtitza	999,79	1,11	30,03	79	100	44	90
Chepelarska	Bachkovo	353,71	1,12	15,03	13	83	0	0
Varbitza	Dgebel	228,85	2,12	12,03	12	23	9	10
Tundga	Kalofer	612,09	1,12	18,03	27	57	12	13
Tundga	Elchovo	95,62	10,12	28,02	12	47	19	3
Mesta	Momina Kula	586,07	8,12	4,03	23	33	0	0
Struma	Krupnik	262,00	1,12	17,02	11	47	0	0

     The area spread of the ice regime's duration is presented on a map (Fig.1), made on the presumption of homogeneity of hydro-thermal conditions in the catchment areas, limited by any two successive hydrometrical gauge stations. The river banks partial ice existence and the full ice cover.

Fig.1.  The duration of the ice regime on the rivers in Bulgaria.

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¹ Geography of Bulgaria, 2002. Geographical Institute with BAS, ForCom, Sofia, 760 p. /in Bulgarian/